Re: [PATCH v8 23/70] mm/mmap: change do_brk_flags() to expand existing VMA and add do_brk_munmap()
From: Liam Howlett
Date: Thu Apr 28 2022 - 20:39:26 EST
* Guenter Roeck <linux@xxxxxxxxxxxx> [220428 16:19]:
> On Tue, Apr 26, 2022 at 03:06:35PM +0000, Liam Howlett wrote:
> > From: "Liam R. Howlett" <Liam.Howlett@xxxxxxxxxx>
> > Avoid allocating a new VMA when it a vma modification can occur. When a
> > brk() can expand or contract a VMA, then the single store operation will
> > only modify one index of the maple tree instead of causing a node to split
> > or coalesce. This avoids unnecessary allocations/frees of maple tree
> > nodes and VMAs.
> > Move some limit & flag verifications out of the do_brk_flags() function to
> > use only relevant checks in the code path of bkr() and vm_brk_flags().
> > Set the vma to check if it can expand in vm_brk_flags() if extra criteria
> > are met.
> > Drop userfaultfd from do_brk_flags() path and only use it in
> > vm_brk_flags() path since that is the only place a munmap will happen.
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> A build failure seen when building ppc64:corenet64_smp_defconfig also
> biscects to this patch.
> mm/mmap.c: In function 'do_brk_flags':
> mm/mmap.c:2908:17: error: implicit declaration of function
> 'khugepaged_enter_vma_merge'; did you mean 'khugepaged_enter_vma'?
> It appears that this is later fixed, but it hurts bisectability
> (and prevents me from finding the actual build failure in linux-next
> when trying to build corenet64_smp_defconfig).
Yeah, that khugepaged_enter_vma_merge was renamed in another patch set.
Andrew made the correction but kept the patch as it was. I think the
suggested change is right.. if you read the commit that introduced
khugepaged_enter_vma(), it seems right at least.
> While looking into the patch, I noticed the following.
> [ ... ]
> > @@ -2773,30 +2888,52 @@ static int do_brk_flags(unsigned long addr, unsigned long len,
> > if (security_vm_enough_memory_mm(mm, len >> PAGE_SHIFT))
> > return -ENOMEM;
> > out:
> > perf_event_mmap(vma);
> > mm->total_vm += len >> PAGE_SHIFT;
> > @@ -2807,18 +2944,21 @@ static int do_brk_flags(unsigned long addr, unsigned long len,
> > validate_mm_mt(mm);
> > return 0;
> > -no_vma_link:
> > vm_area_free(vma);
> That really looks wrong. vm_area_free(vma) can not be reached
> after this patch has been applied.
This came up elsewhere too, Thanks. This shouldn't cause an issue
unless you OOM though.
> > +vma_alloc_fail:
> > + vm_unacct_memory(len >> PAGE_SHIFT);
> > return -ENOMEM;
> > }