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

From: Pavel Begunkov <asml.silence@gmail.com>
To: Johannes Weiner <hannes@cmpxchg.org>,
	Christoph Hellwig <hch@infradead.org>
Cc: Jens Axboe <axboe@kernel.dk>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	linux-fsdevel@vger.kernel.org, io-uring@vger.kernel.org,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH 2/2] block: no-copy bvec for direct IO
Date: Fri, 11 Dec 2020 14:20:11 +0000	[thread overview]
Message-ID: <2404b68a-1569-ce25-c9c4-00d7e42f9e06@gmail.com> (raw)
In-Reply-To: <20201211140622.GA286014@cmpxchg.org>

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