Re: [PATCH v2 0/3] netfs, afs: Fix netfs_write_begin and THP handling

On Thu, Jun 17, 2021 at 09:23:51AM +0100, David Howells wrote:

Here are some patches to fix netfs_write_begin() and the handling of THPs in
that and afs_write_begin/end() in the following ways:

 (1) Use offset_in_thp() rather than manually calculating the offset into
     the page.

 (2) In the future, the len parameter may extend beyond the page allocated.
     This is because the page allocation is deferred to write_begin() and
     that gets to decide what size of THP to allocate.

 (3) In netfs_write_begin(), extract the decision about whether to skip a
     page out to its own helper and have that clear around the region to be
     written, but not clear that region.  This requires the filesystem to
     patch it up afterwards if the hole doesn't get completely filled.

 (4) Due to (3), afs_write_end() now needs to handle short data write into
     the page by generic_perform_write().  I've adopted an analogous
     approach to ceph of just returning 0 in this case and letting the
     caller go round again.

Series looks sane.  I'd like to hear about the thp-related plans in
more detail, but that's a separate story.

I wonder if generic_perform_write() should pass in a flag indicating
whether this is the first attempt or a second attempt at this, and on the
second attempt we just completely prefill the page and just let the partial
write stand - which we have to do if the page was already uptodate when we
started.

Not really - we'll simply get a shorter chunk next time around (with
the patches in -next right now it'll be "the amount we'd actually
managed to copy this time around" in case ->write_begin() tells us
to take a hike), and that shorter chunk is what ->write_begin() will
see.  No need for the flags...