Messages in this thread | | | Date | Mon, 24 Jan 2022 22:55:31 +0000 | From | Wei Yang <> | Subject | Re: [PATCH] mm: remove offset check on page->compound_head and folio->lru |
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On Mon, Jan 24, 2022 at 11:30:10AM +0100, Vlastimil Babka wrote: >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
Thanks for your comment.
I can't imagine the final result. If we would remove page.lru, we could remove FOLIO_MATCH(lru, lru) and add FOLIO_MATCH(compound_head, lru) at that moment?
>thus valid pointers are aligned in such a way they can't accidentaly set the >bit 0. >
-- Wei Yang Help you, Help me
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