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iivbL11QtYRRKBt6zGG1YlEs18W8iAwwNRaeiXfgZOnzVIX1PIWu+nJdwVpkLHi7s8sFJ2oLlPebB3XMoPYqS9VNYjw6giwV2Blwzc+VWfl7G9E73BAtAb1MoMTXRYAeE7VJC4E92CGTnAYXF7kMQsZ1EVbd2kus2wrN0fH6k51HoktlDQMuR2b7lKdZfh3V+uNumlh0JFyb//qXIVCxIzFdtKa2+J5kXYJvNcM2/YxvjMVgiBha1hGkgqmEhZ24ZTS6MUNkhikulLl7DyoppxTlvzc/pjhNEaihBUU7LFRgLHEo= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 4/24/24 9:17 PM, Matthew Wilcox wrote: > On Wed, Apr 24, 2024 at 09:00:50PM -0700, John Hubbard wrote: >>> We want to limit the use of page_mapcount() to places where absolutely >>> required, to prepare for kernel configs where we won't keep track of >>> per-page mapcounts in large folios. >> >> >> Just curious, can you elaborate on the motivation? I probably missed >> the discussions that explained why page_mapcount() in large folios >> is not desirable. Are we getting rid of a field in struct page/folio? >> Some other reason? > > Two reasons. One is that, regardless of anything else, folio_mapcount() > is expensive on large folios as it has to walk every page in the folio > summing the mapcounts. The more important reason is that when we move > to separately allocated folios, we don't want to allocate an array of > mapcounts in order to maintain a per-page mapcount. > > So we're looking for a more compact scheme to avoid maintaining a > per-page mapcount. > I see. Thanks for explaining the story. >>> The khugepage MM selftests keep working as expected, including: >>> >>> Run test: collapse_max_ptes_shared (khugepaged:anon) >>> Allocate huge page... OK >>> Share huge page over fork()... OK >>> Trigger CoW on page 255 of 512... OK >>> Maybe collapse with max_ptes_shared exceeded.... OK >>> Trigger CoW on page 256 of 512... OK >>> Collapse with max_ptes_shared PTEs shared.... OK >>> Check if parent still has huge page... OK >> >> Well, a word of caution! These tests do not (yet) cover either of >> the interesting new cases that folio_likely_mapped_shared() presents: >> KSM or hugetlbfs interactions. In other words, false positives. > > Hmm ... KSM never uses large folios and hugetlbfs is disjoint from > khugepaged? > Oh good. I thought we might have had a testing hole, but no. thanks, -- John Hubbard NVIDIA