From: Jens Axboe <axboe@kernel.dk> mainline inclusion from mainline-v6.10-rc2 commit 9ad6344568cc31ede9741795b3e3c41c21e3156f category: feature bugzilla: https://gitee.com/openeuler/kernel/issues/ICPOEB CVE: NA Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i... -------------------------------- Patch series "Uncached buffered IO", v8. 5 years ago I posted patches adding support for RWF_UNCACHED, as a way to do buffered IO that isn't page cache persistent. The approach back then was to have private pages for IO, and then get rid of them once IO was done. But that then runs into all the issues that O_DIRECT has, in terms of synchronizing with the page cache. So here's a new approach to the same concent, but using the page cache as synchronization. Due to excessive bike shedding on the naming, this is now named RWF_DONTCACHE, and is less special in that it's just page cache IO, except it prunes the ranges once IO is completed. Why do this, you may ask? The tldr is that device speeds are only getting faster, while reclaim is not. Doing normal buffered IO can be very unpredictable, and suck up a lot of resources on the reclaim side. This leads people to use O_DIRECT as a work-around, which has its own set of restrictions in terms of size, offset, and length of IO. It's also inherently synchronous, and now you need async IO as well. While the latter isn't necessarily a big problem as we have good options available there, it also should not be a requirement when all you want to do is read or write some data without caching. Even on desktop type systems, a normal NVMe device can fill the entire page cache in seconds. On the big system I used for testing, there's a lot more RAM, but also a lot more devices. As can be seen in some of the results in the following patches, you can still fill RAM in seconds even when there's 1TB of it. Hence this problem isn't solely a "big hyperscaler system" issue, it's common across the board. Common for both reads and writes with RWF_DONTCACHE is that they use the page cache for IO. Reads work just like a normal buffered read would, with the only exception being that the touched ranges will get pruned after data has been copied. For writes, the ranges will get writeback kicked off before the syscall returns, and then writeback completion will prune the range. Hence writes aren't synchronous, and it's easy to pipeline writes using RWF_DONTCACHE. Folios that aren't instantiated by RWF_DONTCACHE IO are left untouched. This means you that uncached IO will take advantage of the page cache for uptodate data, but not leave anything it instantiated/created in cache. File systems need to support this. This patchset adds support for the generic read path, which covers file systems like ext4. Patches exist to add support for iomap/XFS and btrfs as well, which sit on top of this series. If RWF_DONTCACHE IO is attempted on a file system that doesn't support it, -EOPNOTSUPP is returned. Hence the user can rely on it either working as designed, or flagging and error if that's not the case. The intent here is to give the application a sensible fallback path - eg, it may fall back to O_DIRECT if appropriate, or just live with the fact that uncached IO isn't available and do normal buffered IO. Adding "support" to other file systems should be trivial, most of the time just a one-liner adding FOP_DONTCACHE to the fop_flags in the file_operations struct, if the file system is using either iomap or the generic filemap helpers for reading and writing. Performance results are in patch 8 for reads, and you can find the write side results in the XFS patch adding support for DONTCACHE writes for XFS: https://git.kernel.dk/cgit/linux/commit/?h=buffered-uncached-fs.10&id=257e92... with the tldr being that I see about a 65% improvement in performance for both, with fully predictable IO times. CPU reduction is substantial as well, with no kswapd activity at all for reclaim when using uncached IO. Using it from applications is trivial - just set RWF_DONTCACHE for the read or write, using pwritev2(2) or preadv2(2). For io_uring, same thing, just set RWF_DONTCACHE in sqe->rw_flags for a buffered read/write operation. And that's it. Patches 1..7 are just prep patches, and should have no functional changes at all. Patch 8 adds support for the filemap path for RWF_DONTCACHE reads, and patches 9..12 are just prep patches for supporting the write side of uncached writes. In the below mentioned branch, there are then patches to adopt uncached reads and writes for xfs, btrfs, and ext4. The latter currently relies on bit of a hack for passing whether this is an uncached write or not through ->write_begin(), which can hopefully go away once ext4 adopts iomap for buffered writes. I say this is a hack as it's not the prettiest way to do it, however it is fully solid and will work just fine. Passes full xfstests and fsx overnight runs, no issues observed. That includes the vm running the testing also using RWF_DONTCACHE on the host. I'll post fsstress and fsx patches for RWF_DONTCACHE separately. As far as I'm concerned, no further work needs doing here. And git tree for the patches is here: https://git.kernel.dk/cgit/linux/log/?h=buffered-uncached.10 with the file system patches on top adding support for xfs/btrfs/ext4 here: https://git.kernel.dk/cgit/linux/log/?h=buffered-uncached-fs.10 This patch (of 12): Rather than pass in both the file and position directly from the kiocb, just take a struct kiocb instead. With the kiocb being passed in, skip passing in the address_space separately as well. While doing so, move the ki_flags checking into filemap_create_folio() as well. In preparation for actually needing the kiocb in the function. No functional changes in this patch. Link: https://lkml.kernel.org/r/20241220154831.1086649-1-axboe@kernel.dk Link: https://lkml.kernel.org/r/20241220154831.1086649-2-axboe@kernel.dk Signed-off-by: Jens Axboe <axboe@kernel.dk> Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org> Cc: Brian Foster <bfoster@redhat.com> Cc: Chris Mason <clm@meta.com> Cc: Johannes Weiner <hannes@cmpxchg.org> Cc: Christoph Hellwig <hch@lst.de> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Conflicts: mm/filemap.c [Context conflicts] Signed-off-by: Long Li <leo.lilong@huawei.com> --- mm/filemap.c | 17 +++++++++-------- 1 file changed, 9 insertions(+), 8 deletions(-) diff --git a/mm/filemap.c b/mm/filemap.c index e6f3bf18444e..1103d942835b 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -2489,15 +2489,17 @@ static int filemap_update_page(struct kiocb *iocb, return error; } -static int filemap_create_folio(struct file *file, - struct address_space *mapping, loff_t pos, - struct folio_batch *fbatch) +static int filemap_create_folio(struct kiocb *iocb, struct folio_batch *fbatch) { + struct address_space *mapping = iocb->ki_filp->f_mapping; struct folio *folio; int error; unsigned int min_order = mapping_min_folio_order(mapping); pgoff_t index; + if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_WAITQ)) + return -EAGAIN; + folio = filemap_alloc_folio(mapping_gfp_mask(mapping), min_order); if (!folio) return -ENOMEM; @@ -2516,7 +2518,7 @@ static int filemap_create_folio(struct file *file, * well to keep locking rules simple. */ filemap_invalidate_lock_shared(mapping); - index = (pos >> (PAGE_SHIFT + min_order)) << min_order; + index = (iocb->ki_pos >> (PAGE_SHIFT + min_order)) << min_order; error = filemap_add_folio(mapping, folio, index, mapping_gfp_constraint(mapping, GFP_KERNEL)); if (error == -EEXIST) @@ -2524,7 +2526,8 @@ static int filemap_create_folio(struct file *file, if (error) goto error; - error = filemap_read_folio(file, mapping->a_ops->read_folio, folio); + error = filemap_read_folio(iocb->ki_filp, mapping->a_ops->read_folio, + folio); if (error) goto error; @@ -2575,9 +2578,7 @@ static int filemap_get_pages(struct kiocb *iocb, size_t count, filemap_get_read_batch(mapping, index, last_index - 1, fbatch); } if (!folio_batch_count(fbatch)) { - if (iocb->ki_flags & (IOCB_NOWAIT | IOCB_WAITQ)) - return -EAGAIN; - err = filemap_create_folio(filp, mapping, iocb->ki_pos, fbatch); + err = filemap_create_folio(iocb, fbatch); if (err == AOP_TRUNCATED_PAGE) goto retry; return err; -- 2.39.2