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

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Date:   Fri, 11 Dec 2020 16:38:36 +0100
From:   Johannes Weiner <hannes@...xchg.org>
To:     Pavel Begunkov <asml.silence@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 Fri, Dec 11, 2020 at 02:20:11PM +0000, Pavel Begunkov wrote:
> On 11/12/2020 14:06, Johannes Weiner wrote:
> > 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.)
> 
> I have that stuff fixed, it just didn't go into the RFC. That's basically
> removing replacing add_page() with its version without BIO_WORKINGSET
> in bio_iov_iter_get_pages() and all __bio_iov_*_{add,get}_pages() +
> fix up ./fs/direct-io.c. Should cover all direct cases if I didn't miss
> some.

Ah, that's fantastic! Thanks for clarifying.