linux-kernel - Re: [PATCH v12 00/10] iov_iter: Improve page extraction (pin or just list)

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Date:   Tue, 7 Feb 2023 11:49:32 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>
Cc:     Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Hillf Danton <hdanton@...a.com>, linux-fsdevel@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v12 00/10] iov_iter: Improve page extraction (pin or just
 list)

On 2/7/23 10:12 AM, David Howells wrote:
> Hi Jens, Al, Christoph,
> 
> Here are patches to provide support for extracting pages from an iov_iter
> and to use this in the extraction functions in the block layer bio code.
> 
> The patches make the following changes:
> 
>  (1) Change generic_file_splice_read() to load up an ITER_BVEC iterator
>      with sufficient pages and use that rather than using an ITER_PIPE.
>      This avoids a problem[2] when __iomap_dio_rw() calls iov_iter_revert()
>      to shorten an iterator when it races with truncation.  The reversion
>      causes the pipe iterator to prematurely release the pages it was
>      retaining - despite the read still being in progress.  This caused
>      memory corruption.
> 
>  (2) Remove ITER_PIPE and its paraphernalia as generic_file_splice_read()
>      was the only user.
> 
>  (3) Add a function, iov_iter_extract_pages() to replace
>      iov_iter_get_pages*() that gets refs, pins or just lists the pages as
>      appropriate to the iterator type.
> 
>      Add a function, iov_iter_extract_will_pin() that will indicate from
>      the iterator type how the cleanup is to be performed, returning true
>      if the pages will need unpinning, false otherwise.
> 
>  (4) Make the bio struct carry a pair of flags to indicate the cleanup
>      mode.  BIO_NO_PAGE_REF is replaced with BIO_PAGE_REFFED (indicating
>      FOLL_GET was used) and BIO_PAGE_PINNED (indicating FOLL_PIN was used)
>      is added.
> 
>      BIO_PAGE_REFFED will go away, but at the moment fs/direct-io.c sets it
>      and this series does not fully address that file.
> 
>  (5) Add a function, bio_release_page(), to release a page appropriately to
>      the cleanup mode indicated by the BIO_PAGE_* flags.
> 
>  (6) Make the iter-to-bio code use iov_iter_extract_pages() to retain the
>      pages appropriately and clean them up later.
> 
>  (7) Fix bio_flagged() so that it doesn't prevent a gcc optimisation.

I've updated the for-6.3/iov-extract branch and the for-next branch. This
isn't done to bypass any review, just so we can get some more testing on
this (and because the old one is known broken).

-- 
Jens Axboe