Messages in this thread | | | Date | Mon, 24 Jan 2022 11:30:10 +0100 | Subject | Re: [PATCH] mm: remove offset check on page->compound_head and folio->lru | From | Vlastimil Babka <> |
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On 1/23/22 02:38, Wei Yang wrote: > On Sat, Jan 08, 2022 at 08:13:40AM +0000, Wei Yang wrote: >>On Sat, Jan 08, 2022 at 12:49:53AM +0000, Matthew Wilcox wrote: >>>On Fri, Jan 07, 2022 at 04:08:25PM -0800, Andrew Morton wrote: >>>> On Fri, 7 Jan 2022 22:11:20 +0000 Matthew Wilcox <willy@infradead.org> wrote: >>>> >>>> > > Hi, Matthew >>>> > > >>>> > > Would you mind sharing some insight on this check? >>>> > >>>> > It's right there in the comments. >>>> >>>> Well I can't figure out which comment you're referring to? >>> >>> * WARNING: bit 0 of the first word is used for PageTail(). That >>> * means the other users of this union MUST NOT use the bit to >>> * avoid collision and false-positive PageTail(). >>> >> >>I know this requirement on bit 0 of first word. But I don't see the connection >>between this and the check of page->compound_head and folio->lru. >> >>This is more like a internal requirement on struct page. There are 8 struct in >>this five words union. And this requirement apply to bit 0 of first word of >>all those five struct. >> >>To me, if folio has the same layout of page, folio meets this requirement. I >>still not catch the point why we need this check here. >> > > Hi, Matthew > > Are you back from vocation? If you could give more insight on this check, I > would be appreciated.
I can offer my insight (which might be of course wrong). Ideally one day page.lru will be gone and only folio will be used for LRU pages. Then there won't be a FOLIO_MATCH(lru, lru); and FOLIO_MATCH(compound_head, lru); won't appear to be redundant anymore. lru is list_head so two pointers and thus valid pointers are aligned in such a way they can't accidentaly set the bit 0.
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