On Mon, Dec 9, 2024 at 7:46 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 09.12.24 15:30, Mateusz Guzik wrote: > > On Mon, Dec 9, 2024 at 3:22 PM David Hildenbrand <david@xxxxxxxxxx> wrote: > >> > >> On 09.12.24 13:33, Mateusz Guzik wrote: > >>> That is to say I think this thread just about exhausted the time > >>> warranted by this patch. No hard feelz if it gets dropped, but then I > >>> do strongly suggest adding a justification to the extra load. > >> > >> Maybe it's sufficient for now to simply do your change with a comment: > >> > >> diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h > >> index 8c236c651d1d6..1efc992ad5687 100644 > >> --- a/include/linux/page_ref.h > >> +++ b/include/linux/page_ref.h > >> @@ -234,7 +234,13 @@ static inline bool page_ref_add_unless(struct page *page, int nr, int u) > >> > >> rcu_read_lock(); > >> /* avoid writing to the vmemmap area being remapped */ > >> - if (!page_is_fake_head(page) && page_ref_count(page) != u) > >> + if (!page_is_fake_head(page)) > >> + /* > >> + * atomic_add_unless() will currently never modify the value > >> + * if it already is u. If that ever changes, we'd have to have > >> + * a separate check here, such that we won't be writing to > >> + * write-protected vmemmap areas. > >> + */ > >> ret = atomic_add_unless(&page->_refcount, nr, u); > >> rcu_read_unlock(); > >> > >> > >> It would bail out during testing ... hopefully, such that we can detect any such change. > >> > > > > Not my call to make, but looks good. ;) > > > > fwiw I don't need any credit and I would be more than happy if you > > just submitted the thing as your own without me being mentioned. *No* > > cc would also be appreciated. > > Likely Andrew can add the comment as a fixup. Sorry for not getting back to you sooner: the page_ref_count() test actually is a must, however, I didn't do it in the right order. I only realized my carelessness after Will asked me about it here [1] -- I just posted a fix here [2], CC'ing both of you. If any of you have concerns about it, please let me know in that thread. Otherwise I'll ask Andew to drop this patch. Thank you. [1] https://lore.kernel.org/linux-mm/20241128142028.GA3506@willie-the-truck/ [2] https://lore.kernel.org/linux-mm/20250107043505.351925-1-yuzhao@xxxxxxxxxx/
