On 05/16/2017 10:29 PM, Andrea Arcangeli wrote: > On Wed, Apr 12, 2017 at 03:33:35PM +0200, Vlastimil Babka wrote: >> >> pmdp_invalidate() does: >> >> pmd_t entry = *pmdp; >> set_pmd_at(vma->vm_mm, address, pmdp, pmd_mknotpresent(entry)); >> >> so it's not atomic and if CPU sets dirty or accessed in the middle of >> this, they will be lost? > > I agree it looks like the dirty bit can be lost. Furthermore this also > loses a MMU notifier invalidate that will lead to corruption at the > secondary MMU level (which will keep using the old protection > permission, potentially keeping writing to a wrprotected page). Oh, I didn't paste the whole function, just the pmd manipulation. There's also a third line: flush_pmd_tlb_range(vma, address, address + HPAGE_PMD_SIZE); so there's no missing invalidate, AFAICS? Sorry for the confusion. >> >> But I don't see how the other invalidate caller >> __split_huge_pmd_locked() deals with this either. Andrea, any idea? > > The original code I wrote did this in __split_huge_page_map to create > the "entry" to establish in the pte pagetables: > > entry = mk_pte(page + i, vma->vm_page_prot); > entry = maybe_mkwrite(pte_mkdirty(entry), > vma); > > For anonymous memory the dirty bit is only meaningful for swapping, > and THP couldn't be swapped so setting it unconditional avoided any > issue with the pmdp_invalidate; pmdp_establish. Yeah, but now we are going to swap THP's, and we have shmem THP's... > pmdp_invalidate is needed primarily to avoid aliasing of two different > TLB translation pointing from the same virtual address to the the same > physical address that triggered machine checks (while needing to keep > the pmd huge at all times, back then it was also splitting huge, > splitting is a software bit so userland could still access the data, > splitting bit only blocked kernel code to manipulate on it similar to > what migration entry does right now upstream, except those prevent > userland to access the page during split which is less efficient than > the splitting bit, but at least it's only used for the physical split, > back then there was no difference between virtual and physical split > and physical split is less frequent than the virtual one right now). This took me a while to grasp, but I think I understand now :) > It looks like this needs a pmdp_populate that atomically grabs the > value of the pmd and returns it like pmdp_huge_get_and_clear_notify > does pmdp_huge_get_and_clear_notify() is now gone... > and a _notify variant to use "freeze" is false (if freeze is true > the MMU notifier invalidate must have happened when the pmd was set to > a migration entry). If pmdp_populate_notify (freeze==true) > /pmd_populate (freeze==false) would return the old pmd value > atomically with xchg() (just instead of setting it to 0 we should set > it to the mknotpresent one), then we can set the dirty bit on the ptes > (__split_huge_pmd_locked) or in the pmd itself in the change_huge_pmd > accordingly. I think the confusion was partially caused by the comment at the original caller of pmdp_invalidate(): we first mark the * current pmd notpresent (atomically because here the pmd_trans_huge * and pmd_trans_splitting must remain set at all times on the pmd * until the split is complete for this pmd), It says "atomically" but in fact that only means that the pmd_trans_huge and pmd_trans_splitting flags are not temporarily cleared at any point of time, right? It's not a true atomic swap. > If the "dirty" flag information is obtained by the pmd read before > calling pmdp_invalidate is not ok (losing _notify also not ok). Right. > Thanks! > Andrea > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> > -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
