Messages in this thread | | | Date | Tue, 22 Nov 2022 09:36:57 +0100 | Subject | Re: [PATCH] mm: set the vma flags dirty before testing if it is mergeable | From | David Hildenbrand <> |
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On 22.11.22 09:24, Muhammad Usama Anjum wrote: > The VM_SOFTDIRTY should be set in the vma flags to be tested if new > allocation should be merged in previous vma or not. With this patch, > the new allocations are merged in the previous VMAs. > > I've tested it by reverting the commit 34228d473efe ("mm: ignore > VM_SOFTDIRTY on VMA merging") and after adding this following patch, > I'm seeing that all the new allocations done through mmap() are merged > in the previous VMAs. The number of VMAs doesn't increase drastically > which had contributed to the crash of gimp. If I run the same test after > reverting and not including this patch, the number of VMAs keep on > increasing with every mmap() syscall which proves this patch. > > The commit 34228d473efe ("mm: ignore VM_SOFTDIRTY on VMA merging") > seems like a workaround. But it lets the soft-dirty and non-soft-dirty > VMA to get merged. It helps in avoiding the creation of too many VMAs. > But it creates the problem while adding the feature of clearing the > soft-dirty status of only a part of the memory region. > > Cc: <stable@vger.kernel.org> > Fixes: d9104d1ca966 ("mm: track vma changes with VM_SOFTDIRTY bit") > Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com> > --- > We need more testing of this patch. > > While implementing clear soft-dirty bit for a range of address space, I'm > facing an issue. The non-soft dirty VMA gets merged sometimes with the soft > dirty VMA. Thus the non-soft dirty VMA become dirty which is undesirable. > When discussed with the some other developers they consider it the > regression. Why the non-soft dirty page should appear as soft dirty when it > isn't soft dirty in reality? I agree with them. Should we revert > 34228d473efe or find a workaround in the IOCTL? > > * Revert may cause the VMAs to expand in uncontrollable situation where the > soft dirty bit of a lot of memory regions or the whole address space is > being cleared again and again. AFAIK normal process must either be only > clearing a few memory regions. So the applications should be okay. There is > still chance of regressions if some applications are already using the > soft-dirty bit. I'm not sure how to test it. > > * Add a flag in the IOCTL to ignore the dirtiness of VMA. The user will > surely lose the functionality to detect reused memory regions. But the > extraneous soft-dirty pages would not appear. I'm trying to do this in the > patch series [1]. Some discussion is going on that this fails with some > mprotect use case [2]. I still need to have a look at the mprotect selftest > to see how and why this fails. I think this can be implemented after some > more work probably in mprotect side. > > [1] https://lore.kernel.org/all/20221109102303.851281-1-usama.anjum@collabora.com/ > [2] https://lore.kernel.org/all/bfcae708-db21-04b4-0bbe-712badd03071@redhat.com/ > --- > mm/mmap.c | 21 +++++++++++---------- > 1 file changed, 11 insertions(+), 10 deletions(-) > > diff --git a/mm/mmap.c b/mm/mmap.c > index f9b96b387a6f..6934b8f61fdc 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -1708,6 +1708,15 @@ unsigned long mmap_region(struct file *file, unsigned long addr, > vm_flags |= VM_ACCOUNT; > } > > + /* > + * New (or expanded) vma always get soft dirty status. > + * Otherwise user-space soft-dirty page tracker won't > + * be able to distinguish situation when vma area unmapped, > + * then new mapped in-place (which must be aimed as > + * a completely new data area). > + */ > + vm_flags |= VM_SOFTDIRTY; > + > /* > * Can we just expand an old mapping? > */ > @@ -1823,15 +1832,6 @@ unsigned long mmap_region(struct file *file, unsigned long addr, > if (file) > uprobe_mmap(vma); > > - /* > - * New (or expanded) vma always get soft dirty status. > - * Otherwise user-space soft-dirty page tracker won't > - * be able to distinguish situation when vma area unmapped, > - * then new mapped in-place (which must be aimed as > - * a completely new data area). > - */ > - vma->vm_flags |= VM_SOFTDIRTY; > - > vma_set_page_prot(vma);
vma_set_page_prot(vma) has to be called after adjusting vma->vm_flags.
Did not look into the details here, but that jumped at me.
-- Thanks,
David / dhildenb
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