Messages in this thread | | | Date | Thu, 28 Apr 2022 15:06:13 +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 4/28/2022 1:55 PM, 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.
Right, this is intended. In the original code, if it is not a shared PMD, we will use flush_cache_page() to do cache flushing. However the flush_cache_page() can not cover the whole size of a hugetlb page on some architectures, which is fixed by patch 3.
> 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?
IMHO after patch 3, the code will be changed as below, so seems no need to separate the validation of the shared PMDs from huge_pmd_unshare() into a new function.
if (folio_test_hugetlb(folio)) { flush_cache_range(vma, range.start, range.end);
if (!folio_test_anon(folio)) { VM_BUG_ON(!(flags & TTU_RMAP_LOCKED));
if (hugetlb_pmd_shared(mm, vma, pvmw.pte)) { huge_pmd_unshare(mm, vma, &address, pvmw.pte)); flush_tlb_range(vma, range.start, range.end); ...... break; } } } else { ...... }
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