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

[Al Viro]

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...