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