linux-kernel - Re: [PATCH -next] mm/filemap: fix that first page is not mark accessed in filemap

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Date:   Thu, 2 Jun 2022 11:22:48 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yu Kuai <yukuai3@...wei.com>
Cc:     <willy@...radead.org>, <kent.overstreet@...il.com>,
        <axboe@...nel.dk>, <linux-fsdevel@...r.kernel.org>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <yi.zhang@...wei.com>
Subject: Re: [PATCH -next] mm/filemap: fix that first page is not mark
 accessed in filemap_read()

On Thu, 2 Jun 2022 16:21:29 +0800 Yu Kuai <yukuai3@...wei.com> wrote:

> In filemap_read(), 'ra->prev_pos' is set to 'iocb->ki_pos + copied',
> while it should be 'iocb->ki_ops'. For consequence,
> folio_mark_accessed() will not be called for 'fbatch.folios[0]' since
> 'iocb->ki_pos' is always equal to 'ra->prev_pos'.
> 
> ...
>
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2728,10 +2728,11 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
>  				flush_dcache_folio(folio);
>  
>  			copied = copy_folio_to_iter(folio, offset, bytes, iter);
> -
> -			already_read += copied;
> -			iocb->ki_pos += copied;
> -			ra->prev_pos = iocb->ki_pos;
> +			if (copied) {
> +				ra->prev_pos = iocb->ki_pos;
> +				already_read += copied;
> +				iocb->ki_pos += copied;
> +			}
>  
>  			if (copied < bytes) {
>  				error = -EFAULT;

It seems tidier, but does it matter?  If copied==0 we're going to break
out and return -EFAULT anyway?