linux-kernel - Re: [RFC] mm: optimise generic_file_read

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Date:   Fri, 6 Aug 2021 13:48:38 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC] mm: optimise generic_file_read_iter

On Fri, Aug 06, 2021 at 12:42:43PM +0100, Pavel Begunkov wrote:
> Unless direct I/O path of generic_file_read_iter() ended up with an
> error or a short read, it doesn't use inode. So, load inode and size
> later, only when they're needed. This cuts two memory reads and also
> imrpoves code generation, e.g. loads from stack.

... and the same question here.

> NOTE: as a side effect, it reads inode->i_size after ->direct_IO(), and
> I'm not sure whether that's valid, so would be great to get feedback
> from someone who knows better.

Ought to be safe, I think, but again, how much effect have you observed
from the patch?