Messages in this thread | | | Subject | Re: [v2 PATCH 2/2 -mm] mm: brk: dwongrade mmap_sem to read when shrinking | From | Yang Shi <> | Date | Thu, 27 Sep 2018 09:06:29 -0700 |
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On 9/27/18 5:50 AM, Kirill A. Shutemov wrote: > On Thu, Sep 27, 2018 at 02:10:34AM +0800, Yang Shi wrote: >> brk might be used to shinrk memory mapping too other than munmap(). > s/shinrk/shrink/ > >> So, it may hold write mmap_sem for long time when shrinking large >> mapping, as what commit ("mm: mmap: zap pages with read mmap_sem in >> munmap") described. >> >> The brk() will not manipulate vmas anymore after __do_munmap() call for >> the mapping shrink use case. But, it may set mm->brk after >> __do_munmap(), which needs hold write mmap_sem. >> >> However, a simple trick can workaround this by setting mm->brk before >> __do_munmap(). Then restore the original value if __do_munmap() fails. >> With this trick, it is safe to downgrade to read mmap_sem. >> >> So, the same optimization, which downgrades mmap_sem to read for >> zapping pages, is also feasible and reasonable to this case. >> >> The period of holding exclusive mmap_sem for shrinking large mapping >> would be reduced significantly with this optimization. >> >> Cc: Michal Hocko <mhocko@kernel.org> >> Cc: Kirill A. Shutemov <kirill@shutemov.name> >> Cc: Matthew Wilcox <willy@infradead.org> >> Cc: Laurent Dufour <ldufour@linux.vnet.ibm.com> >> Cc: Vlastimil Babka <vbabka@suse.cz> >> Cc: Andrew Morton <akpm@linux-foundation.org> >> Signed-off-by: Yang Shi <yang.shi@linux.alibaba.com> >> --- >> v2: Rephrase the commit per Michal >> >> mm/mmap.c | 40 ++++++++++++++++++++++++++++++---------- >> 1 file changed, 30 insertions(+), 10 deletions(-) >> >> diff --git a/mm/mmap.c b/mm/mmap.c >> index 017bcfa..0d2fae1 100644 >> --- a/mm/mmap.c >> +++ b/mm/mmap.c >> @@ -193,9 +193,11 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long >> unsigned long retval; >> unsigned long newbrk, oldbrk; >> struct mm_struct *mm = current->mm; >> + unsigned long origbrk = mm->brk; > Is it safe to read mm->brk outside the lock?
Aha, thanks for catching this. It can be moved inside down_write().
Will solve in the next version.
Thanks, Yang
> >> struct vm_area_struct *next; >> unsigned long min_brk; >> bool populate; >> + bool downgrade = false; > Again, > > s/downgrade/downgraded/ ? > >> LIST_HEAD(uf); >> >> if (down_write_killable(&mm->mmap_sem)) >> @@ -229,14 +231,29 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long >> >> newbrk = PAGE_ALIGN(brk); >> oldbrk = PAGE_ALIGN(mm->brk); >> - if (oldbrk == newbrk) >> - goto set_brk; >> + if (oldbrk == newbrk) { >> + mm->brk = brk; >> + goto success; >> + } >> >> - /* Always allow shrinking brk. */ >> + /* >> + * Always allow shrinking brk. >> + * __do_munmap() may downgrade mmap_sem to read. >> + */ >> if (brk <= mm->brk) { >> - if (!do_munmap(mm, newbrk, oldbrk-newbrk, &uf)) >> - goto set_brk; >> - goto out; >> + /* >> + * mm->brk need to be protected by write mmap_sem, update it >> + * before downgrading mmap_sem. >> + * When __do_munmap fail, it will be restored from origbrk. >> + */ >> + mm->brk = brk; >> + retval = __do_munmap(mm, newbrk, oldbrk-newbrk, &uf, true); >> + if (retval < 0) { >> + mm->brk = origbrk; >> + goto out; >> + } else if (retval == 1) >> + downgrade = true; >> + goto success; >> } >> >> /* Check against existing mmap mappings. */ >> @@ -247,18 +264,21 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long >> /* Ok, looks good - let it rip. */ >> if (do_brk_flags(oldbrk, newbrk-oldbrk, 0, &uf) < 0) >> goto out; >> - >> -set_brk: >> mm->brk = brk; >> + >> +success: >> populate = newbrk > oldbrk && (mm->def_flags & VM_LOCKED) != 0; >> - up_write(&mm->mmap_sem); >> + if (downgrade) >> + up_read(&mm->mmap_sem); >> + else >> + up_write(&mm->mmap_sem); >> userfaultfd_unmap_complete(mm, &uf); >> if (populate) >> mm_populate(oldbrk, newbrk - oldbrk); >> return brk; >> >> out: >> - retval = mm->brk; >> + retval = origbrk; >> up_write(&mm->mmap_sem); >> return retval; >> } >> -- >> 1.8.3.1 >>
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