linux-kernel - Re: [PATCH 2/6] block: add dio_w_*() wrappers for pin, unpin user pages

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Message-ID: <d89943aa-5528-a424-099f-4b1a2151b893@nvidia.com>
Date:   Sat, 27 Aug 2022 17:31:32 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Miklos Szeredi <miklos@...redi.hu>,
        Christoph Hellwig <hch@...radead.org>,
        "Darrick J . Wong" <djwong@...nel.org>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna@...nel.org>, Jan Kara <jack@...e.cz>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/6] block: add dio_w_*() wrappers for pin, unpin user
 pages

On 8/27/22 17:12, Andrew Morton wrote:
> On Sat, 27 Aug 2022 16:59:32 -0700 John Hubbard <jhubbard@...dia.com> wrote:
> 
>> Anyway, I'll change my patch locally for now, to this:
>>
>> static inline void dio_w_unpin_user_pages(struct page **pages,
>> 					  unsigned long npages)
>> {
>> 	/* Careful, release_pages() uses a smaller integer type for npages: */
>> 	if (WARN_ON_ONCE(npages > (unsigned long)INT_MAX))
>> 		return;
>>
>> 	release_pages(pages, (int)npages);
>> }
> 
> Well, it might be slower.  release_pages() has a ton of fluff.
> 
> As mentioned, the above might be faster if the pages tend
> to have page_count()==1??
> 

I don't think we can know the answer to that. This code is called
in all kinds of situations, and it seems to me that whatever
tradeoffs are best for release_pages(), are probably also reasonable
for this code.

Even with all the fluff in release_pages(), it at least batches
the pages, as opposed to the simple put_page() in a loop. Most of 
the callers do have more than one page to release in non-error cases,
so release_pages() does seem better.


thanks,

-- 
John Hubbard
NVIDIA