On Thu, Jun 15, 2017 at 06:35:21AM +0530, Aneesh Kumar K.V wrote: > > > On Wednesday 14 June 2017 10:25 PM, Will Deacon wrote: > > Hi Aneesh, > > > > On Wed, Jun 14, 2017 at 08:55:26PM +0530, Aneesh Kumar K.V wrote: > > > On Wednesday 14 June 2017 07:21 PM, Kirill A. Shutemov wrote: > > > > Vlastimil noted that pmdp_invalidate() is not atomic and we can loose > > > > dirty and access bits if CPU sets them after pmdp dereference, but > > > > before set_pmd_at(). > > > > > > > > The bug doesn't lead to user-visible misbehaviour in current kernel, but > > > > fixing this would be critical for future work on THP: both huge-ext4 and THP > > > > swap out rely on proper dirty tracking. > > > > > > > > Unfortunately, there's no way to address the issue in a generic way. We need to > > > > fix all architectures that support THP one-by-one. > > > > > > > > All architectures that have THP supported have to provide atomic > > > > pmdp_invalidate(). If generic implementation of pmdp_invalidate() is used, > > > > architecture needs to provide atomic pmdp_mknonpresent(). > > > > > > > > I've fixed the issue for x86, but I need help with the rest. > > > > > > > > So far THP is supported on 8 architectures. Power and S390 already provides > > > > atomic pmdp_invalidate(). x86 is fixed by this patches, so 5 architectures > > > > left: > > > > > > > > - arc; > > > > - arm; > > > > - arm64; > > > > - mips; > > > > - sparc -- it has custom pmdp_invalidate(), but it's racy too; > > > > > > > > Please, help me with them. > > > > > > > > Kirill A. Shutemov (3): > > > > x86/mm: Provide pmdp_mknotpresent() helper > > > > mm: Do not loose dirty and access bits in pmdp_invalidate() > > > > mm, thp: Do not loose dirty bit in __split_huge_pmd_locked() > > > > > > > > > > > > > But in __split_huge_pmd_locked() we collected the dirty bit early. So even > > > if we made pmdp_invalidate() atomic, if we had marked the pmd pte entry > > > dirty after we collected the dirty bit, we still loose it right ? > > > > > > > > > May be we should relook at pmd PTE udpate interface. We really need an > > > interface that can update pmd entries such that we don't clear it in > > > between. IMHO, we can avoid the pmdp_invalidate() completely, if we can > > > switch from a pmd PTE entry to a pointer to PTE page (pgtable_t). We also > > > need this interface to avoid the madvise race fixed by > > > > There's a good chance I'm not following your suggestion here, but it's > > probably worth me pointing out that swizzling a page table entry from a > > block mapping (e.g. a huge page mapped at the PMD level) to a table entry > > (e.g. a pointer to a page of PTEs) can lead to all sorts of horrible > > problems on ARM, including amalgamation of TLB entries and fatal aborts. > > > > So we really need to go via an invalid entry, with appropriate TLB > > invalidation before installing the new entry. > > > > I am not suggesting we don't do the invalidate (the need for that is > documented in __split_huge_pmd_locked(). I am suggesting we need a new > interface, something like Andrea suggested. > > old_pmd = pmdp_establish(pmd_mknotpresent()); > > instead of pmdp_invalidate(). We can then use this in scenarios where we > want to update pmd PTE entries, where right now we go through a pmdp_clear > and set_pmd path. We should really not do that for THP entries. Which cases are you talking about? When do we need to clear pmd and set later? -- Kirill A. Shutemov
