On Wed, Jan 05, 2022 at 12:17:46AM -0800, John Hubbard wrote:
> > + if (!folio_test_dirty(folio)) {
> > + folio_lock(folio);
> > + folio_mark_dirty(folio);
> > + folio_unlock(folio);
>
> At some point, maybe even here, I suspect that creating the folio
> version of set_page_dirty_lock() would help. I'm sure you have
> a better feel for whether it helps, after doing all of this conversion
> work, but it just sort of jumped out at me as surprising to see it
> in this form.
I really hate set_page_dirty_lock(). It smacks of "there is a locking
rule here which we're violating, so we'll just take the lock to fix it"
without understanding why there's a locking problem here.
As far as I can tell, originally, the intent was that you would lock
the page before modifying any of the data in the page. ie you would
do:
gup()
lock_page()
addr = kmap_page()
*addr = 1;
kunmap_page()
set_page_dirty()
unlock_page()
put_page()
and that would prevent races between modifying the page and (starting)
writeback, not to mention truncate() and various other operations.
Clearly we can't do that for DMA-pinned pages. There's only one lock
bit. But do we even need to take the lock if we have the page pinned?
What are we protecting against?
If it's truncate(), I think we must already have protection against
that as we could easily have:
pup()
truncate()
lock_page()
set_page_dirty()
unlock_page()
unpin
