linux-kernel - Re: [PATCH 05/15] mm: allow read-ahead with IOCB

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Message-ID: <20200624010253.GB5369@dread.disaster.area>
Date:   Wed, 24 Jun 2020 11:02:53 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Add@...r.kernel.org, support@...r.kernel.org, for@...r.kernel.org,
        async@...r.kernel.org, buffered@...r.kernel.org,
        reads@...r.kernel.org
Cc:     io-uring@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, Jens Axboe <axboe@...nel.dk>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [PATCH 05/15] mm: allow read-ahead with IOCB_NOWAIT set

On Thu, Jun 18, 2020 at 08:43:45AM -0600, Jens Axboe wrote:
> The read-ahead shouldn't block, so allow it to be done even if
> IOCB_NOWAIT is set in the kiocb.
> 
> Acked-by: Johannes Weiner <hannes@...xchg.org>
> Signed-off-by: Jens Axboe <axboe@...nel.dk>
> ---
>  mm/filemap.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index f0ae9a6308cb..3378d4fca883 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2028,8 +2028,6 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
>  
>  		page = find_get_page(mapping, index);
>  		if (!page) {
> -			if (iocb->ki_flags & IOCB_NOWAIT)
> -				goto would_block;
>  			page_cache_sync_readahead(mapping,
>  					ra, filp,
>  					index, last_index - index);

Doesn't think break preadv2(RWF_NOWAIT) semantics for on buffered
reads? i.e. this can now block on memory allocation for the page
cache, which is something RWF_NOWAIT IO should not do....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com