At this point, we're handling the majority of file-backed page faults under the VMA lock, using the ->map_pages entry point. This patch set attempts to expand that for the following siutations:
- We have to do a read. This could be because we've hit the point in the readahead window where we need to kick off the next readahead, or because the page is simply not present in cache. - We're handling a write fault. Most applications don't do I/O by writes to shared mmaps for very good reasons, but some do, and it'd be nice to not make that slow unnecessarily. - We're doing a COW of a private mapping (both PTE already present and PTE not-present). These are two different codepaths and I handle both of them in this patch set.
There is no support in this patch set for drivers to mark themselves as being VMA lock friendly; they could implement the ->map_pages vm_operation, but if they do, they would be the first. This is probably something we want to change at some point in the future, and I've marked where to make that change in the code.
There is very little performance change in the benchmarks we've run; mostly because the vast majority of page faults are handled through the other paths. I still think this patch series is useful for workloads that may take these paths more often, and just for cleaning up the fault path in general (it's now clearer why we have to retry in these cases).
Matthew Wilcox (Oracle) (6): mm: make lock_folio_maybe_drop_mmap() VMA lock aware mm: call wp_page_copy() under the VMA lock mm: handle shared faults under the VMA lock mm: handle COW faults under the VMA lock mm: handle read faults under the VMA lock mm: handle write faults to RO pages under the VMA lock
mm/filemap.c | 13 ++++---- mm/memory.c | 93 ++++++++++++++++++++++++++++++++-------------------- 2 files changed, 65 insertions(+), 41 deletions(-)
From: "Matthew Wilcox (Oracle)" willy@infradead.org
mainline inclusion from mainline-v6.7-rc1 commit 5d74b2ab2c15d596c470bae6626f345d5575a9d0 category: other bugzilla: https://gitee.com/openeuler/kernel/issues/I8K7C7
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
--------------------------------
Patch series "Handle more faults under the VMA lock", v2.
At this point, we're handling the majority of file-backed page faults under the VMA lock, using the ->map_pages entry point. This patch set attempts to expand that for the following siutations:
- We have to do a read. This could be because we've hit the point in the readahead window where we need to kick off the next readahead, or because the page is simply not present in cache. - We're handling a write fault. Most applications don't do I/O by writes to shared mmaps for very good reasons, but some do, and it'd be nice to not make that slow unnecessarily. - We're doing a COW of a private mapping (both PTE already present and PTE not-present). These are two different codepaths and I handle both of them in this patch set.
There is no support in this patch set for drivers to mark themselves as being VMA lock friendly; they could implement the ->map_pages vm_operation, but if they do, they would be the first. This is probably something we want to change at some point in the future, and I've marked where to make that change in the code.
There is very little performance change in the benchmarks we've run; mostly because the vast majority of page faults are handled through the other paths. I still think this patch series is useful for workloads that may take these paths more often, and just for cleaning up the fault path in general (it's now clearer why we have to retry in these cases).
This patch (of 6):
Drop the VMA lock instead of the mmap_lock if that's the one which is held.
Link: https://lkml.kernel.org/r/20231006195318.4087158-1-willy@infradead.org Link: https://lkml.kernel.org/r/20231006195318.4087158-2-willy@infradead.org Signed-off-by: Matthew Wilcox (Oracle) willy@infradead.org Reviewed-by: Suren Baghdasaryan surenb@google.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Tong Tiangen tongtiangen@huawei.com --- mm/filemap.c | 13 +++++++------ 1 file changed, 7 insertions(+), 6 deletions(-)
diff --git a/mm/filemap.c b/mm/filemap.c index f0a15ce1bd1b..1c398edcfcaf 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -3104,7 +3104,7 @@ static int lock_folio_maybe_drop_mmap(struct vm_fault *vmf, struct folio *folio,
/* * NOTE! This will make us return with VM_FAULT_RETRY, but with - * the mmap_lock still held. That's how FAULT_FLAG_RETRY_NOWAIT + * the fault lock still held. That's how FAULT_FLAG_RETRY_NOWAIT * is supposed to work. We have way too many special cases.. */ if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT) @@ -3114,13 +3114,14 @@ static int lock_folio_maybe_drop_mmap(struct vm_fault *vmf, struct folio *folio, if (vmf->flags & FAULT_FLAG_KILLABLE) { if (__folio_lock_killable(folio)) { /* - * We didn't have the right flags to drop the mmap_lock, - * but all fault_handlers only check for fatal signals - * if we return VM_FAULT_RETRY, so we need to drop the - * mmap_lock here and return 0 if we don't have a fpin. + * We didn't have the right flags to drop the + * fault lock, but all fault_handlers only check + * for fatal signals if we return VM_FAULT_RETRY, + * so we need to drop the fault lock here and + * return 0 if we don't have a fpin. */ if (*fpin == NULL) - mmap_read_unlock(vmf->vma->vm_mm); + release_fault_lock(vmf); return 0; } } else
From: "Matthew Wilcox (Oracle)" willy@infradead.org
mainline inclusion from mainline-v6.7-rc1 commit 164b06f238b986317131e6b61b2f22aabcbc2cc0 category: other bugzilla: https://gitee.com/openeuler/kernel/issues/I8K7C7
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
--------------------------------
It is usually safe to call wp_page_copy() under the VMA lock. The only unsafe situation is when no anon_vma has been allocated for this VMA, and we have to look at adjacent VMAs to determine if their anon_vma can be shared. Since this happens only for the first COW of a page in this VMA, the majority of calls to wp_page_copy() do not need to fall back to the mmap_sem.
Add vmf_anon_prepare() as an alternative to anon_vma_prepare() which will return RETRY if we currently hold the VMA lock and need to allocate an anon_vma. This lets us drop the check in do_wp_page().
Link: https://lkml.kernel.org/r/20231006195318.4087158-3-willy@infradead.org Signed-off-by: Matthew Wilcox (Oracle) willy@infradead.org Reviewed-by: Suren Baghdasaryan surenb@google.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Tong Tiangen tongtiangen@huawei.com --- mm/memory.c | 39 ++++++++++++++++++++++++++------------- 1 file changed, 26 insertions(+), 13 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c index f4667bfae83b..73cbca4ed65d 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3044,6 +3044,21 @@ static inline void wp_page_reuse(struct vm_fault *vmf, struct folio *folio) count_vm_event(PGREUSE); }
+static vm_fault_t vmf_anon_prepare(struct vm_fault *vmf) +{ + struct vm_area_struct *vma = vmf->vma; + + if (likely(vma->anon_vma)) + return 0; + if (vmf->flags & FAULT_FLAG_VMA_LOCK) { + vma_end_read(vma); + return VM_FAULT_RETRY; + } + if (__anon_vma_prepare(vma)) + return VM_FAULT_OOM; + return 0; +} + /* * Handle the case of a page which we actually need to copy to a new page, * either due to COW or unsharing. @@ -3071,27 +3086,29 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf) pte_t entry; int page_copied = 0; struct mmu_notifier_range range; - int ret; + vm_fault_t ret;
delayacct_wpcopy_start();
if (vmf->page) old_folio = page_folio(vmf->page); - if (unlikely(anon_vma_prepare(vma))) - goto oom; + ret = vmf_anon_prepare(vmf); + if (unlikely(ret)) + goto out;
if (is_zero_pfn(pte_pfn(vmf->orig_pte))) { new_folio = vma_alloc_zeroed_movable_folio(vma, vmf->address); if (!new_folio) goto oom; } else { + int err; new_folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma, vmf->address, false); if (!new_folio) goto oom;
- ret = __wp_page_copy_user(&new_folio->page, vmf->page, vmf); - if (ret) { + err = __wp_page_copy_user(&new_folio->page, vmf->page, vmf); + if (err) { /* * COW failed, if the fault was solved by other, * it's fine. If not, userspace would re-fault on @@ -3104,7 +3121,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf) folio_put(old_folio);
delayacct_wpcopy_end(); - return ret == -EHWPOISON ? VM_FAULT_HWPOISON : 0; + return err == -EHWPOISON ? VM_FAULT_HWPOISON : 0; } kmsan_copy_page_meta(&new_folio->page, vmf->page); } @@ -3214,11 +3231,13 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf) oom_free_new: folio_put(new_folio); oom: + ret = VM_FAULT_OOM; +out: if (old_folio) folio_put(old_folio);
delayacct_wpcopy_end(); - return VM_FAULT_OOM; + return ret; }
/** @@ -3444,12 +3463,6 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf) return 0; } copy: - if ((vmf->flags & FAULT_FLAG_VMA_LOCK) && !vma->anon_vma) { - pte_unmap_unlock(vmf->pte, vmf->ptl); - vma_end_read(vmf->vma); - return VM_FAULT_RETRY; - } - /* * Ok, we need to copy. Oh, well.. */
From: "Matthew Wilcox (Oracle)" willy@infradead.org
mainline inclusion from mainline-v6.7-rc1 commit 4ed4379881aa62588aba6442a9f362a8cf7624e6 category: other bugzilla: https://gitee.com/openeuler/kernel/issues/I8K7C7
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
--------------------------------
There are many implementations of ->fault and some of them depend on mmap_lock being held. All vm_ops that implement ->map_pages() end up calling filemap_fault(), which I have audited to be sure it does not rely on mmap_lock. So (for now) key off ->map_pages existing as a flag to indicate that it's safe to call ->fault while only holding the vma lock.
Link: https://lkml.kernel.org/r/20231006195318.4087158-4-willy@infradead.org Signed-off-by: Matthew Wilcox (Oracle) willy@infradead.org Reviewed-by: Suren Baghdasaryan surenb@google.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Tong Tiangen tongtiangen@huawei.com --- mm/memory.c | 22 ++++++++++++++++++---- 1 file changed, 18 insertions(+), 4 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c index 73cbca4ed65d..f5ac37abdb06 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3044,6 +3044,21 @@ static inline void wp_page_reuse(struct vm_fault *vmf, struct folio *folio) count_vm_event(PGREUSE); }
+/* + * We could add a bitflag somewhere, but for now, we know that all + * vm_ops that have a ->map_pages have been audited and don't need + * the mmap_lock to be held. + */ +static inline vm_fault_t vmf_can_call_fault(const struct vm_fault *vmf) +{ + struct vm_area_struct *vma = vmf->vma; + + if (vma->vm_ops->map_pages || !(vmf->flags & FAULT_FLAG_VMA_LOCK)) + return 0; + vma_end_read(vma); + return VM_FAULT_RETRY; +} + static vm_fault_t vmf_anon_prepare(struct vm_fault *vmf) { struct vm_area_struct *vma = vmf->vma; @@ -4655,10 +4670,9 @@ static vm_fault_t do_shared_fault(struct vm_fault *vmf) vm_fault_t ret, tmp; struct folio *folio;
- if (vmf->flags & FAULT_FLAG_VMA_LOCK) { - vma_end_read(vma); - return VM_FAULT_RETRY; - } + ret = vmf_can_call_fault(vmf); + if (ret) + return ret;
ret = __do_fault(vmf); if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
From: "Matthew Wilcox (Oracle)" willy@infradead.org
mainline inclusion from mainline-v6.7-rc1 commit 4de8c93a4751e10737b6af65db42c743228c67a6 category: other bugzilla: https://gitee.com/openeuler/kernel/issues/I8K7C7
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
--------------------------------
If the page is not currently present in the page tables, we need to call the page fault handler to find out which page we're supposed to COW, so we need to both check that there is already an anon_vma and that the fault handler doesn't need the mmap_lock.
Link: https://lkml.kernel.org/r/20231006195318.4087158-5-willy@infradead.org Signed-off-by: Matthew Wilcox (Oracle) willy@infradead.org Reviewed-by: Suren Baghdasaryan surenb@google.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Tong Tiangen tongtiangen@huawei.com --- mm/memory.c | 12 +++++------- 1 file changed, 5 insertions(+), 7 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c index f5ac37abdb06..f4ef386be88f 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -4625,13 +4625,11 @@ static vm_fault_t do_cow_fault(struct vm_fault *vmf) struct vm_area_struct *vma = vmf->vma; vm_fault_t ret;
- if (vmf->flags & FAULT_FLAG_VMA_LOCK) { - vma_end_read(vma); - return VM_FAULT_RETRY; - } - - if (unlikely(anon_vma_prepare(vma))) - return VM_FAULT_OOM; + ret = vmf_can_call_fault(vmf); + if (!ret) + ret = vmf_anon_prepare(vmf); + if (ret) + return ret;
vmf->cow_page = alloc_page_vma(GFP_HIGHUSER_MOVABLE, vma, vmf->address); if (!vmf->cow_page)
From: "Matthew Wilcox (Oracle)" willy@infradead.org
mainline inclusion from mainline-v6.7-rc1 commit 12214eba1992642eee5813a9cc9f626e5b2d1815 category: other bugzilla: https://gitee.com/openeuler/kernel/issues/I8K7C7
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
--------------------------------
Most file-backed faults are already handled through ->map_pages(), but if we need to do I/O we'll come this way. Since filemap_fault() is now safe to be called under the VMA lock, we can handle these faults under the VMA lock now.
Link: https://lkml.kernel.org/r/20231006195318.4087158-6-willy@infradead.org Signed-off-by: Matthew Wilcox (Oracle) willy@infradead.org Reviewed-by: Suren Baghdasaryan surenb@google.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Tong Tiangen tongtiangen@huawei.com --- mm/memory.c | 7 +++---- 1 file changed, 3 insertions(+), 4 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c index f4ef386be88f..f1399e9f527d 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -4603,10 +4603,9 @@ static vm_fault_t do_read_fault(struct vm_fault *vmf) return ret; }
- if (vmf->flags & FAULT_FLAG_VMA_LOCK) { - vma_end_read(vmf->vma); - return VM_FAULT_RETRY; - } + ret = vmf_can_call_fault(vmf); + if (ret) + return ret;
ret = __do_fault(vmf); if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY)))
From: "Matthew Wilcox (Oracle)" willy@infradead.org
from mainline-v6.7-rc1 commit 4a68fef16df9d88d528094116f8bbd2dbfa62089 category: other bugzilla: https://gitee.com/openeuler/kernel/issues/I8K7C7
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
--------------------------------
I think this is a pretty rare occurrence, but for consistency handle faults with the VMA lock held the same way that we handle other faults with the VMA lock held.
Link: https://lkml.kernel.org/r/20231006195318.4087158-7-willy@infradead.org Signed-off-by: Matthew Wilcox (Oracle) willy@infradead.org Reviewed-by: Suren Baghdasaryan surenb@google.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Tong Tiangen tongtiangen@huawei.com --- mm/memory.c | 13 ++++++------- 1 file changed, 6 insertions(+), 7 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c index f1399e9f527d..e1a0eb8b776a 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -3304,10 +3304,9 @@ static vm_fault_t wp_pfn_shared(struct vm_fault *vmf) vm_fault_t ret;
pte_unmap_unlock(vmf->pte, vmf->ptl); - if (vmf->flags & FAULT_FLAG_VMA_LOCK) { - vma_end_read(vmf->vma); - return VM_FAULT_RETRY; - } + ret = vmf_can_call_fault(vmf); + if (ret) + return ret;
vmf->flags |= FAULT_FLAG_MKWRITE; ret = vma->vm_ops->pfn_mkwrite(vmf); @@ -3331,10 +3330,10 @@ static vm_fault_t wp_page_shared(struct vm_fault *vmf, struct folio *folio) vm_fault_t tmp;
pte_unmap_unlock(vmf->pte, vmf->ptl); - if (vmf->flags & FAULT_FLAG_VMA_LOCK) { + tmp = vmf_can_call_fault(vmf); + if (tmp) { folio_put(folio); - vma_end_read(vmf->vma); - return VM_FAULT_RETRY; + return tmp; }
tmp = do_page_mkwrite(vmf, folio);
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