On 1/8/22 20:39, Matthew Wilcox wrote:
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?
This is a fun question, because you're asking it at a point when the
overall problem remains unsolved. That is, the interaction between
file-backed pages and gup/pup is still completely broken.
And I don't have an answer for you: it does seem like lock_page() is
completely pointless here. Looking back, there are some 25 callers of
unpin_user_pages_dirty_lock(), and during all those patch reviews, no
one noticed this point!
Anyway...so maybe most or even all of the unpin_user_pages_dirty_lock()
callers do not really need a _lock variant, after all.
Or, maybe it really is required and we're overlooking some subtle
filesystem-related point. It would be nice to get a second look from
Christoph Hellwig and Jan Kara (+CC).
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
I think truncate vs. gup/pup() is no more and and no less broken in this
regard than it's ever been. We need another LSF/MM conference with some
more shouting, and/or file leases, for that. :)
thanks,
--
John Hubbard
NVIDIA
