linux-kernel - Re: [PATCH 01/67] mm: Stop filemap_read() from grabbing a superfluous page

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Date:   Tue, 19 Oct 2021 13:13:04 -0400
From:   Jeff Layton <jlayton@...hat.com>
To:     David Howells <dhowells@...hat.com>, linux-cachefs@...hat.com
Cc:     Kent Overstreet <kent.overstreet@...il.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        Trond Myklebust <trondmy@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Steve French <sfrench@...ba.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Omar Sandoval <osandov@...ndov.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-afs@...ts.infradead.org, linux-nfs@...r.kernel.org,
        linux-cifs@...r.kernel.org, ceph-devel@...r.kernel.org,
        v9fs-developer@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/67] mm: Stop filemap_read() from grabbing a
 superfluous page

On Mon, 2021-10-18 at 15:50 +0100, David Howells wrote:
> Under some circumstances, filemap_read() will allocate sufficient pages to
> read to the end of the file, call readahead/readpages on them and copy the
> data over - and then it will allocate another page at the EOF and call
> readpage on that and then ignore it.  This is unnecessary and a waste of
> time and resources.
> 
> filemap_read() *does* check for this, but only after it has already done
> the allocation and I/O.  Fix this by checking before calling
> filemap_get_pages() also.
> 
> Signed-off-by: David Howells <dhowells@...hat.com>
> Acked-by: Kent Overstreet <kent.overstreet@...il.com>
> cc: Matthew Wilcox (Oracle) <willy@...radead.org>
> cc: linux-mm@...ck.org
> cc: linux-fsdevel@...r.kernel.org
> Link: https://lore.kernel.org/r/160588481358.3465195.16552616179674485179.stgit@warthog.procyon.org.uk/
> ---
> 
>  mm/filemap.c |    4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index dae481293b5d..c0cdc44c844e 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2625,6 +2625,10 @@ ssize_t filemap_read(struct kiocb *iocb, struct iov_iter *iter,
>  		if ((iocb->ki_flags & IOCB_WAITQ) && already_read)
>  			iocb->ki_flags |= IOCB_NOWAIT;
>  
> +		isize = i_size_read(inode);
> +		if (unlikely(iocb->ki_pos >= isize))
> +			goto put_pages;
> +
>  		error = filemap_get_pages(iocb, iter, &pvec);
>  		if (error < 0)
>  			break;
> 
> 

I would wager that it's worth checking for this. I imagine read calls
beyond EOF are common enough that it's probably helpful to optimize that
case:

Acked-by: Jeff Layton <jlayton@...hat.com>