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 UvnNzG82 85kBbGg4n0vlVkCOFfLSiQXMN01Y8ougaOoxFoqJnoe14zsl21ImcpA0gf28ILpEBeWDC6zBL3FmOdSRU0g3U6oVbm+idHqjnjl04vOg02UhY8K2SIFph0Cnib5MI5JsYhj/UlTrJ4IQh2QSIkUJ48ngdJME4hYrxMf6F9eYgxqs0A6mUgQw4PsOSkdyLF1laqvtM6Ad1oXYjqALrBECO7qoYLn3AmBrSsERrDSGivTP6tjbbMX/pIhqxl1NoZ7aAsDcHECTEB02dyU5Lx0igN11rYPu05ePryMhyCf6ML4QyNsZd2WmJYckzoOBBWTTFcqbKoADaqDEC4OyG/qQxiO8JL7+Z9hBxms0iVjByjvRSh+oBasl6KsjsXKD5PCNmzkLbx6NWcptl7JfDkCJnT10uyPsaZVqj1o20oe7N99/4xrNfy2wL+hphfdBwmglyNHOwY5RjyUE1bIPC6xEiv4rNvwbfH+A1Yv7A/RqZ1xkB5Y8= 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: On 06/09/23 21:59, Tarun Sahu wrote: > folio_set_order(folio, 0) is used in kernel at two places > __destroy_compound_gigantic_folio and __prep_compound_gigantic_folio. > Currently, It is called to clear out the folio->_folio_nr_pages and > folio->_folio_order. > > For __destroy_compound_gigantic_folio: > In past, folio_set_order(folio, 0) was needed because page->mapping used > to overlap with _folio_nr_pages and _folio_order. So if these fields were > left uncleared during freeing gigantic hugepages, they were causing > "BUG: bad page state" due to non-zero page->mapping. Now, After > Commit a01f43901cfb ("hugetlb: be sure to free demoted CMA pages to > CMA") page->mapping has explicitly been cleared out for tail pages. Also, > _folio_order and _folio_nr_pages no longer overlaps with page->mapping. > > So, folio_set_order(folio, 0) can be removed from freeing gigantic > folio path (__destroy_compound_gigantic_folio). > > Another place, folio_set_order(folio, 0) is called inside > __prep_compound_gigantic_folio during error path. Here, > folio_set_order(folio, 0) can also be removed if we move > folio_set_order(folio, order) after for loop. > > The patch also moves _folio_set_head call in __prep_compound_gigantic_folio() > such that we avoid clearing them in the error path. > > Also, as Mike pointed out: > "It would actually be better to move the calls _folio_set_head and > folio_set_order in __prep_compound_gigantic_folio() as suggested here. Why? > In the current code, the ref count on the 'head page' is still 1 (or more) > while those calls are made. So, someone could take a speculative ref on the > page BEFORE the tail pages are set up." > > This way, folio_set_order(folio, 0) is no more needed. And it will also > helps removing the confusion of folio order being set to 0 (as _folio_order > field is part of first tail page). > > Testing: I have run LTP tests, which all passes. and also I have written > the test in LTP which tests the bug caused by compound_nr and page->mapping > overlapping. > > https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/mem/hugetlb/hugemmap/hugemmap32.c > > Running on older kernel ( < 5.10-rc7) with the above bug this fails while > on newer kernel and, also with this patch it passes. > > Signed-off-by: Tarun Sahu > --- > v2->v3 > - removed the copy of page/folio definition from commit msg > v1->v2 > - Reword the commit message > > mm/hugetlb.c | 9 +++------ > mm/internal.h | 8 ++------ > 2 files changed, 5 insertions(+), 12 deletions(-) Thanks for answering all the questions along the way! Reviewed-by: Mike Kravetz -- Mike Kravetz