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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: o9dfzfh5nzc7yta4xibe64ppukdhkqf3 X-Rspam-User: X-Rspamd-Queue-Id: D5B47180018 X-Rspamd-Server: rspam08 X-HE-Tag: 1742558830-615390 X-HE-Meta: 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 tMYPhTx7 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, 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 3/21/25 9:33 PM, Vlastimil Babka wrote: > On 3/21/25 12:25, Gavin Shan wrote: >> On 3/21/25 8:11 PM, David Hildenbrand wrote: >>> On 21.03.25 10:23, Vlastimil Babka wrote: >>>> On 3/21/25 06:31, Gavin Shan wrote: >>>>> Found by code inspection. There are two places where the parameter >>>>> passed to page_mapcount_is_type() is (page->__mapcount), which is >>>>> correct since it should be one more than the value, as explained in >>>>> the comments to page_mapcount_is_type(): (a) page_has_type() in >>>>> page-flags.h (b) __dump_folio() in mm/debug.c >>>> >>>> IIUC you are right. Luckily thanks to the the PGTY_mapcount_underflow limit, >>>> this off-by-one error doesn't currently cause visible issues i.e. >>>> misclassifications legitimate mapcount as page type and vice versa, right? >>>> We'd have to have a mapcount underflown severely right to the limit to make >>>> that off-by-one error cross it? >>> >>> Agreed. Likely not stable material because it isn't actually fixing anything (because of the safety gaps). >>> >> >> Yes, it shouldn't cause any visible impacts so far due to the gap. > > Thanks for confirming, please state that in the commit log/cover letter too. > Yes, the commit log and cover letter has been improved for this in v2. >> I just found the issue by code inspection. Lets drop the fix tags >> in v2. > > Fixes: tag is fine and correct, just Cc: stable is unnecessary. > Thanks for the hints. The 'Cc: stable' tag has been dropped, but the 'Fixes:' tag is kept in v2, which was posted. https://lore.kernel.org/linux-mm/20250321120222.1456770-1-gshan@redhat.com/T/#t >>>> >>>> I wonder if a more future-proof solution would be to redefine >>>> page_mapcount_is_type() instead to not subtract. But I'll leave that to willy. >>> >>> With upcoming changes around that, likely best to leave that alone. I expect page_mapcount_is_type() to completely vanish. >>> >> >> +1 to remove page_mapcount_is_type(). After Willy confirms, I can post >> an extra series to do it if needed. >> Thanks, Gavin