Messages in this thread | | | Date | Wed, 4 May 2022 10:50:33 +0800 | Subject | Re: [PATCH v2 2/3] mm: rmap: Move the cache flushing to the correct place for hugetlb PMD sharing | From | Baolin Wang <> |
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On 5/4/2022 2:42 AM, Mike Kravetz wrote: > On 4/27/22 22:55, Muchun Song wrote: >> On Wed, Apr 27, 2022 at 06:52:06PM +0800, Baolin Wang wrote: >>> The cache level flush will always be first when changing an existing >>> virtual–>physical mapping to a new value, since this allows us to >>> properly handle systems whose caches are strict and require a >>> virtual–>physical translation to exist for a virtual address. So we >>> should move the cache flushing before huge_pmd_unshare(). >>> >> >> Right. >> >>> As Muchun pointed out[1], now the architectures whose supporting hugetlb >>> PMD sharing have no cache flush issues in practice. But I think we >>> should still follow the cache/TLB flushing rules when changing a valid >>> virtual address mapping in case of potential issues in future. >> >> Right. One point i need to clarify. I do not object this change but >> want you to clarify this (not an issue in practice) in commit log >> to let others know they do not need to bp this. >> >>> >>> [1] https://lore.kernel.org/all/YmT%2F%2FhuUbFX+KHcy@FVFYT0MHHV2J.usts.net/ >>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com> >>> --- >>> mm/rmap.c | 40 ++++++++++++++++++++++------------------ >>> 1 file changed, 22 insertions(+), 18 deletions(-) >>> >>> diff --git a/mm/rmap.c b/mm/rmap.c >>> index 61e63db..4f0d115 100644 >>> --- a/mm/rmap.c >>> +++ b/mm/rmap.c >>> @@ -1535,15 +1535,16 @@ static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma, >>> * do this outside rmap routines. >>> */ >>> VM_BUG_ON(!(flags & TTU_RMAP_LOCKED)); >>> + /* >>> + * huge_pmd_unshare may unmap an entire PMD page. >>> + * There is no way of knowing exactly which PMDs may >>> + * be cached for this mm, so we must flush them all. >>> + * start/end were already adjusted above to cover this >>> + * range. >>> + */ >>> + flush_cache_range(vma, range.start, range.end); >>> + >> >> flush_cache_range() is always called even if we do not need to flush. >> How about introducing a new helper like hugetlb_pmd_shared() which >> returns true for shared PMD? Then: >> >> if (hugetlb_pmd_shared(mm, vma, pvmw.pte)) { >> flush_cache_range(vma, range.start, range.end); >> huge_pmd_unshare(mm, vma, &address, pvmw.pte); >> flush_tlb_range(vma, range.start, range.end); >> } >> >> The code could be a little simpler. Right? >> >> Thanks. >> > > I thought about adding a 'hugetlb_pmd_shared()' interface for another use. > I believe it could even be used earlier in this call sequence. Since we > hold i_mmap_rwsem, we would even test for shared BEFORE calling > adjust_range_if_pmd_sharing_possible. We can not make an authoritative test > in adjust range... because not all callers will be holding i_mmap_rwsem. > > I think we COULD optimize to minimize the flush range. However, I think > that would complicate this code even more, and it is difficult enough to > follow. > > My preference would be to over flush as is done here for correctness and > simplification. We can optimize later if desired.
OK. Agree.
> > With Muchun's comment that this is not an issue in practice today, > Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Thanks.
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