linux-kernel - Re: [PATCH 2/2] block: no-copy bvec for direct IO

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Message-ID: <20201211140622.GA286014@cmpxchg.org>
Date:   Fri, 11 Dec 2020 15:06:22 +0100
From:   Johannes Weiner <hannes@...xchg.org>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Pavel Begunkov <asml.silence@...il.com>,
        Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH 2/2] block: no-copy bvec for direct IO

On Wed, Dec 09, 2020 at 08:40:05AM +0000, Christoph Hellwig wrote:
> > +	/*
> > +	 * In practice groups of pages tend to be accessed/reclaimed/refaulted
> > +	 * together. To not go over bvec for those who didn't set BIO_WORKINGSET
> > +	 * approximate it by looking at the first page and inducing it to the
> > +	 * whole bio
> > +	 */
> > +	if (unlikely(PageWorkingset(iter->bvec->bv_page)))
> > +		bio_set_flag(bio, BIO_WORKINGSET);
> 
> IIRC the feedback was that we do not need to deal with BIO_WORKINGSET
> at all for direct I/O.

Yes, this hunk is incorrect. We must not use this flag for direct IO.
It's only for paging IO, when you bring in the data at page->mapping +
page->index. Otherwise you tell the pressure accounting code that you
are paging in a thrashing page, when really you're just reading new
data into a page frame that happens to be hot.

(As per the other thread, bio_add_page() currently makes that same
mistake for direct IO. I'm fixing that.)