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Message-ID: <ZzOEVwWpGEaq6wE7@bfoster> Date: Tue, 12 Nov 2024 11:37:43 -0500 From: Brian Foster <bfoster@...hat.com> To: Jens Axboe <axboe@...nel.dk> Cc: linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, hannes@...xchg.org, clm@...a.com, linux-kernel@...r.kernel.org, willy@...radead.org, kirill@...temov.name, linux-btrfs@...r.kernel.org, linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org Subject: Re: [PATCH 13/16] iomap: make buffered writes work with RWF_UNCACHED On Mon, Nov 11, 2024 at 04:37:40PM -0700, Jens Axboe wrote: > Add iomap buffered write support for RWF_UNCACHED. If RWF_UNCACHED is > set for a write, mark the folios being written with drop_writeback. Then s/drop_writeback/uncached/ ? BTW, this might be getting into wonky "don't care that much" territory, but something else to be aware of is that certain writes can potentially change pagecache state as a side effect outside of the actual buffered write itself. For example, xfs calls iomap_zero_range() on write extension (i.e. pos > isize), which uses buffered writes and thus could populate a pagecache folio without setting it uncached, even if done on behalf of an uncached write. I've only made a first pass and could be missing some details, but IIUC I _think_ this means something like writing out a stream of small, sparse and file extending uncached writes could actually end up behaving more like sync I/O. Again, not saying that's something we really care about, just raising it in case it's worth considering or documenting.. Brian > writeback completion will drop the pages. The write_iter handler simply > kicks off writeback for the pages, and writeback completion will take > care of the rest. > > This still needs the user of the iomap buffered write helpers to call > iocb_uncached_write() upon successful issue of the writes. > > Signed-off-by: Jens Axboe <axboe@...nel.dk> > --- > fs/iomap/buffered-io.c | 15 +++++++++++++-- > include/linux/iomap.h | 4 +++- > 2 files changed, 16 insertions(+), 3 deletions(-) > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c > index ef0b68bccbb6..2f2a5db04a68 100644 > --- a/fs/iomap/buffered-io.c > +++ b/fs/iomap/buffered-io.c > @@ -603,6 +603,8 @@ struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos, size_t len) > > if (iter->flags & IOMAP_NOWAIT) > fgp |= FGP_NOWAIT; > + if (iter->flags & IOMAP_UNCACHED) > + fgp |= FGP_UNCACHED; > fgp |= fgf_set_order(len); > > return __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT, > @@ -1023,8 +1025,9 @@ ssize_t > iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i, > const struct iomap_ops *ops, void *private) > { > + struct address_space *mapping = iocb->ki_filp->f_mapping; > struct iomap_iter iter = { > - .inode = iocb->ki_filp->f_mapping->host, > + .inode = mapping->host, > .pos = iocb->ki_pos, > .len = iov_iter_count(i), > .flags = IOMAP_WRITE, > @@ -1034,9 +1037,14 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i, > > if (iocb->ki_flags & IOCB_NOWAIT) > iter.flags |= IOMAP_NOWAIT; > + if (iocb->ki_flags & IOCB_UNCACHED) > + iter.flags |= IOMAP_UNCACHED; > > - while ((ret = iomap_iter(&iter, ops)) > 0) > + while ((ret = iomap_iter(&iter, ops)) > 0) { > + if (iocb->ki_flags & IOCB_UNCACHED) > + iter.iomap.flags |= IOMAP_F_UNCACHED; > iter.processed = iomap_write_iter(&iter, i); > + } > > if (unlikely(iter.pos == iocb->ki_pos)) > return ret; > @@ -1770,6 +1778,9 @@ static int iomap_add_to_ioend(struct iomap_writepage_ctx *wpc, > size_t poff = offset_in_folio(folio, pos); > int error; > > + if (folio_test_uncached(folio)) > + wpc->iomap.flags |= IOMAP_F_UNCACHED; > + > if (!wpc->ioend || !iomap_can_add_to_ioend(wpc, pos)) { > new_ioend: > error = iomap_submit_ioend(wpc, 0); > diff --git a/include/linux/iomap.h b/include/linux/iomap.h > index f61407e3b121..2efc72df19a2 100644 > --- a/include/linux/iomap.h > +++ b/include/linux/iomap.h > @@ -64,6 +64,7 @@ struct vm_fault; > #define IOMAP_F_BUFFER_HEAD 0 > #endif /* CONFIG_BUFFER_HEAD */ > #define IOMAP_F_XATTR (1U << 5) > +#define IOMAP_F_UNCACHED (1U << 6) > > /* > * Flags set by the core iomap code during operations: > @@ -173,8 +174,9 @@ struct iomap_folio_ops { > #define IOMAP_NOWAIT (1 << 5) /* do not block */ > #define IOMAP_OVERWRITE_ONLY (1 << 6) /* only pure overwrites allowed */ > #define IOMAP_UNSHARE (1 << 7) /* unshare_file_range */ > +#define IOMAP_UNCACHED (1 << 8) /* uncached IO */ > #ifdef CONFIG_FS_DAX > -#define IOMAP_DAX (1 << 8) /* DAX mapping */ > +#define IOMAP_DAX (1 << 9) /* DAX mapping */ > #else > #define IOMAP_DAX 0 > #endif /* CONFIG_FS_DAX */ > -- > 2.45.2 > >
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