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

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Date:   Fri, 11 Dec 2020 14:20:11 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Johannes Weiner <hannes@...xchg.org>,
        Christoph Hellwig <hch@...radead.org>
Cc:     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 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.

-- 
Pavel Begunkov