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

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Message-ID: <20200624014645.GJ21350@casper.infradead.org>
Date:   Wed, 24 Jun 2020 02:46:45 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Add@...r.kernel.org, support@...r.kernel.org, for@...r.kernel.org,
        async@...r.kernel.org, buffered@...r.kernel.org,
        reads@...r.kernel.org, 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 Wed, Jun 24, 2020 at 11:02:53AM +1000, Dave Chinner wrote:
> 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.
> 
> 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....

Yes.  This eventually ends up in page_cache_readahead_unbounded()
which gets its gfp flags from readahead_gfp_mask(mapping).

I'd be quite happy to add a gfp_t to struct readahead_control.
The other thing I've been looking into for other reasons is adding
a memalloc_nowait_{save,restore}, which would avoid passing down
the gfp_t.