linux-kernel - Re: [PATCH] iomap: fix short copy in iomap_write

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Message-ID: <CAASaF6xHTv6iZd5ttHOJ_M=hpjaGZOnUCGSHkbGy_yLbe2G8nw@mail.gmail.com>
Date:   Wed, 18 Oct 2023 16:32:19 +0200
From:   Jan Stancek <jstancek@...hat.com>
To:     Christoph Hellwig <hch@....de>
Cc:     djwong@...nel.org, willy@...radead.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        viro@...iv.linux.org.uk
Subject: Re: [PATCH] iomap: fix short copy in iomap_write_iter()

On Wed, Oct 18, 2023 at 2:22 PM Christoph Hellwig <hch@....de> wrote:
>
> On Wed, Oct 18, 2023 at 10:24:20AM +0200, Jan Stancek wrote:
> > Make next iteration retry with amount of bytes we managed to copy.
>
> The observation and logic fix look good.  But I wonder if simply
> using a goto instead of the extra variable would be a tad cleaner?
> Something like this?

Looks good to me. Would you be OK if I re-posted it as v2 with your
Signed-off-by added?


>
> ---
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index 061f3d14c12001..2d491590795aa4 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -881,8 +881,10 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
>                 size_t bytes;           /* Bytes to write to folio */
>                 size_t copied;          /* Bytes copied from user */
>
> +               bytes = iov_iter_count(i);
> +retry:
>                 offset = pos & (chunk - 1);
> -               bytes = min(chunk - offset, iov_iter_count(i));
> +               bytes = min(chunk - offset, bytes);
>                 status = balance_dirty_pages_ratelimited_flags(mapping,
>                                                                bdp_flags);
>                 if (unlikely(status))
> @@ -933,10 +935,12 @@ static loff_t iomap_write_iter(struct iomap_iter *iter, struct iov_iter *i)
>                          * halfway through, might be a race with munmap,
>                          * might be severe memory pressure.
>                          */
> -                       if (copied)
> -                               bytes = copied;
>                         if (chunk > PAGE_SIZE)
>                                 chunk /= 2;
> +                       if (copied) {
> +                               bytes = copied;
> +                               goto retry;
> +                       }
>                 } else {
>                         pos += status;
>                         written += status;
>