From: Lorenzo Stoakes lstoakes@gmail.com
mainline inclusion from mainline-6.7-rc1 commit 6facf36ee49675ea952713a7a1488e9f191e7eec category: feature bugzilla: https://gitee.com/openeuler/kernel/issues/I98AW9 CVE: NA
-------------------------------------------------
The existing comments in filemap_fault() suggest that, after either a minor fault has occurred and filemap_get_folio() found a folio in the page cache, or a major fault arose and __filemap_get_folio(FGP_CREATE...) did the job (having relied on do_sync_mmap_readahead() or filemap_read_folio() to read in the folio), the only possible reason it could not be uptodate is because of an error.
This is not so, as if, for instance, the fault occurred within a VMA which had the VM_RAND_READ flag set (via madvise() with the MADV_RANDOM flag specified), this would cause even synchronous readahead to fail to read in the folio.
I confirmed this by dropping page caches and faulting in memory madvise()'d this way, observing that this code path was reached on each occasion.
Clarify the comments to include this case, and additionally update the comment recently added around the invalidate lock logic to make it clear the comment explicitly refers to the minor fault case.
In addition, while we're here, refer to folios rather than pages.
[lstoakes@gmail.com: correct identation as per Christopher's feedback] Link: https://lkml.kernel.org/r/2c7014c0-6343-4e76-8697-3f84f54350bd@lucifer.local Link: https://lkml.kernel.org/r/20230930231029.88196-1-lstoakes@gmail.com Signed-off-by: Lorenzo Stoakes lstoakes@gmail.com Reviewed-by: Jan Kara jack@suse.cz Reviewed-by: Christoph Hellwig hch@lst.de Cc: Matthew Wilcox (Oracle) willy@infradead.org Signed-off-by: Andrew Morton akpm@linux-foundation.org (cherry picked from commit 6facf36ee49675ea952713a7a1488e9f191e7eec) Signed-off-by: Kefeng Wang wangkefeng.wang@huawei.com --- mm/filemap.c | 19 +++++++++++++------ 1 file changed, 13 insertions(+), 6 deletions(-)
diff --git a/mm/filemap.c b/mm/filemap.c index bf4c07bded63..1022bce6bcd5 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -3319,21 +3319,28 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) VM_BUG_ON_FOLIO(!folio_contains(folio, index), folio);
/* - * We have a locked page in the page cache, now we need to check - * that it's up-to-date. If not, it is going to be due to an error. + * We have a locked folio in the page cache, now we need to check + * that it's up-to-date. If not, it is going to be due to an error, + * or because readahead was otherwise unable to retrieve it. */ if (unlikely(!folio_test_uptodate(folio))) { /* - * The page was in cache and uptodate and now it is not. - * Strange but possible since we didn't hold the page lock all - * the time. Let's drop everything get the invalidate lock and - * try again. + * If the invalidate lock is not held, the folio was in cache + * and uptodate and now it is not. Strange but possible since we + * didn't hold the page lock all the time. Let's drop + * everything, get the invalidate lock and try again. */ if (!mapping_locked) { folio_unlock(folio); folio_put(folio); goto retry_find; } + + /* + * OK, the folio is really not uptodate. This can be because the + * VMA has the VM_RAND_READ flag set, or because an error + * arose. Let's read it in directly. + */ goto page_not_uptodate; }