On Wed, Oct 13, 2021 at 02:42:42PM -0700, Yang Shi wrote: > On Tue, Oct 12, 2021 at 8:41 PM Peter Xu <peterx@xxxxxxxxxx> wrote: > > > > On Tue, Oct 12, 2021 at 08:27:06PM -0700, Yang Shi wrote: > > > > But this also reminded me that shouldn't we be with the page lock already > > > > during the process of "setting hwpoison-subpage bit, split thp, clear > > > > hwpoison-subpage bit"? If it's only the small window that needs protection, > > > > while when looking up the shmem pagecache we always need to take the page lock > > > > too, then it seems already safe even without the extra bit? Hmm? > > > > > > I don't quite get your point. Do you mean memory_failure()? If so the > > > answer is no, outside the page lock. And the window may be indefinite > > > since file THP doesn't get split before this series and the split may > > > fail even after this series. > > > > What I meant is that we could extend the page_lock in try_to_split_thp_page() > > to cover setting hwpoison-subpage too (and it of course covers the clearing if > > thp split succeeded, as that's part of the split process). But yeah it's a > > good point that the split may fail, so the extra bit seems still necessary. > > > > Maybe that'll be something worth mentioning in the commit message too? The > > commit message described very well on the overhead of looping over 512 pages, > > however the reader can easily overlook the real reason for needing this bit - > > IMHO it's really for the thp split failure case, as we could also mention that > > if thp split won't fail, page lock should be suffice (imho). We could also > > Not only for THP split failure case. Before this series, shmem THP > does't get split at all. And this commit is supposed to be backported > to the older versions, so saying "page lock is sufficient" is not > precise and confusing. Sure, please feel free to use any wording you prefer as long as the other side of the lock besides the performance impact could be mentioned. Thanks, -- Peter Xu
