Messages in this thread | | | Subject | Re: [RFC v7 PATCH 4/4] mm: unmap special vmas with regular do_munmap() | From | Yang Shi <> | Date | Fri, 10 Aug 2018 10:00:45 -0700 |
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On 8/10/18 3:46 AM, Vlastimil Babka wrote: > On 08/10/2018 01:36 AM, Yang Shi wrote: >> Unmapping vmas, which have VM_HUGETLB | VM_PFNMAP flag set or >> have uprobes set, need get done with write mmap_sem held since >> they may update vm_flags. >> >> So, it might be not safe enough to deal with these kind of special >> mappings with read mmap_sem. Deal with such mappings with regular >> do_munmap() call. >> >> Michal suggested to make this as a separate patch for safer and more >> bisectable sake. >> >> Cc: Michal Hocko <mhocko@kernel.org> >> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com> >> --- >> mm/mmap.c | 24 ++++++++++++++++++++++++ >> 1 file changed, 24 insertions(+) >> >> diff --git a/mm/mmap.c b/mm/mmap.c >> index 2234d5a..06cb83c 100644 >> --- a/mm/mmap.c >> +++ b/mm/mmap.c >> @@ -2766,6 +2766,16 @@ static inline void munlock_vmas(struct vm_area_struct *vma, >> } >> } >> >> +static inline bool can_zap_with_rlock(struct vm_area_struct *vma) >> +{ >> + if ((vma->vm_file && >> + vma_has_uprobes(vma, vma->vm_start, vma->vm_end)) | > vma_has_uprobes() seems to be rather expensive check with e.g. > unconditional spinlock. uprobe_munmap() seems to have some precondition > cheaper checks for e.g. cases when there's no uprobes in the system > (should be common?).
I think they are common, i.e. checking vm prot since uprobes are typically installed for VM_EXEC vmas. We could use those checks to save some cycles.
> > BTW, uprobe_munmap() touches mm->flags, not vma->flags, so it should be > evaluated more carefully for being called under mmap sem for reading, as > having vmas already detached is no guarantee.
We might just leave uprobe vmas to use regular do_munmap? I'm supposed they should be not very common. And, uprobes just can be installed for VM_EXEC vma, although there may be large text segments, typically VM_EXEC vmas are unmapped when process exits, so the latency might be fine.
> >> + (vma->vm_flags | (VM_HUGETLB | VM_PFNMAP))) > ^ I think replace '|' with '&' here?
Yes, thanks for catching this.
> >> + return false; >> + >> + return true; >> +} >> + >> /* >> * Zap pages with read mmap_sem held >> * >> @@ -2808,6 +2818,17 @@ static int do_munmap_zap_rlock(struct mm_struct *mm, unsigned long start, >> goto out; >> } >> >> + /* >> + * Unmapping vmas, which have VM_HUGETLB | VM_PFNMAP flag set or >> + * have uprobes set, need get done with write mmap_sem held since >> + * they may update vm_flags. Deal with such mappings with regular >> + * do_munmap() call. >> + */ >> + for (vma = start_vma; vma && vma->vm_start < end; vma = vma->vm_next) { >> + if (!can_zap_with_rlock(vma)) >> + goto regular_path; >> + } >> + >> /* Handle mlocked vmas */ >> if (mm->locked_vm) { >> vma = start_vma; >> @@ -2828,6 +2849,9 @@ static int do_munmap_zap_rlock(struct mm_struct *mm, unsigned long start, >> >> return 0; >> >> +regular_path: > I think it's missing a down_write_* here.
No, the jump is called before downgrade_write.
Thanks, Yang
> >> + ret = do_munmap(mm, start, len, uf); >> + >> out: >> up_write(&mm->mmap_sem); >> return ret; >>
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