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Message-ID: <20251009150125.GD6188@frogsfrogsfrogs>
Date: Thu, 9 Oct 2025 08:01:25 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: alexjlzheng@...il.com, dave.hansen@...ux.intel.com
Cc: brauner@...nel.org, linux-xfs@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
dave.hansen@...ux.intel.com,
Jinliang Zheng <alexjlzheng@...cent.com>
Subject: Re: [PATCH] iomap: move prefaulting out of hot write path
On Thu, Oct 09, 2025 at 05:08:51PM +0800, alexjlzheng@...il.com wrote:
> From: Jinliang Zheng <alexjlzheng@...cent.com>
>
> Prefaulting the write source buffer incurs an extra userspace access
> in the common fast path. Make iomap_write_iter() consistent with
> generic_perform_write(): only touch userspace an extra time when
> copy_folio_from_iter_atomic() has failed to make progress.
>
> This patch is inspired by commit 665575cff098 ("filemap: move
> prefaulting out of hot write path").
Seems fine to me, but I wonder if dhansen has any thoughts about this
patch ... which exactly mirrors one he sent eight months ago?
--D
> Signed-off-by: Jinliang Zheng <alexjlzheng@...cent.com>
> ---
> fs/iomap/buffered-io.c | 31 ++++++++++++++++---------------
> 1 file changed, 16 insertions(+), 15 deletions(-)
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 8b847a1e27f1..6e6573fce78a 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -972,21 +972,6 @@ static int iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i,
> if (bytes > iomap_length(iter))
> bytes = iomap_length(iter);
>
> - /*
> - * Bring in the user page that we'll copy from _first_.
> - * Otherwise there's a nasty deadlock on copying from the
> - * same page as we're writing to, without it being marked
> - * up-to-date.
> - *
> - * For async buffered writes the assumption is that the user
> - * page has already been faulted in. This can be optimized by
> - * faulting the user page.
> - */
> - if (unlikely(fault_in_iov_iter_readable(i, bytes) == bytes)) {
> - status = -EFAULT;
> - break;
> - }
> -
> status = iomap_write_begin(iter, write_ops, &folio, &offset,
> &bytes);
> if (unlikely(status)) {
> @@ -1001,6 +986,12 @@ static int iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i,
> if (mapping_writably_mapped(mapping))
> flush_dcache_folio(folio);
>
> + /*
> + * Faults here on mmap()s can recurse into arbitrary
> + * filesystem code. Lots of locks are held that can
> + * deadlock. Use an atomic copy to avoid deadlocking
> + * in page fault handling.
> + */
> copied = copy_folio_from_iter_atomic(folio, offset, bytes, i);
> written = iomap_write_end(iter, bytes, copied, folio) ?
> copied : 0;
> @@ -1039,6 +1030,16 @@ static int iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i,
> bytes = copied;
> goto retry;
> }
> +
> + /*
> + * 'folio' is now unlocked and faults on it can be
> + * handled. Ensure forward progress by trying to
> + * fault it in now.
> + */
> + if (fault_in_iov_iter_readable(i, bytes) == bytes) {
> + status = -EFAULT;
> + break;
> + }
> } else {
> total_written += written;
> iomap_iter_advance(iter, &written);
> --
> 2.49.0
>
>
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