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Sun, 27 Aug 2023 02:09:22 -0700 (PDT) Date: Sun, 27 Aug 2023 10:09:21 +0100 From: Lorenzo Stoakes To: "Joel Fernandes (Google)" Cc: linux-kernel@vger.kernel.org, Linus Torvalds , linux-kselftest@vger.kernel.org, linux-mm@kvack.org, Shuah Khan , Vlastimil Babka , Michal Hocko , Kirill A Shutemov , "Liam R. Howlett" , "Paul E. McKenney" , Suren Baghdasaryan , Kalesh Singh , Lokesh Gidra Subject: Re: [PATCH v5 1/7] mm/mremap: Optimize the start addresses in move_page_tables() Message-ID: <58e63c5e-e043-4651-bf6d-9fc52f78cce6@lucifer.local> References: <20230822015501.791637-1-joel@joelfernandes.org> <20230822015501.791637-2-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230822015501.791637-2-joel@joelfernandes.org> X-Rspam-User: X-Stat-Signature: gtuej5bxdtxu6ms43qnabwiprycyxk63 X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 1D0E41C0018 X-HE-Tag: 1693127364-588129 X-HE-Meta: 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 bjYilMU5 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Aug 22, 2023 at 01:54:54AM +0000, Joel Fernandes (Google) wrote: > Recently, we see reports [1] of a warning that triggers due to > move_page_tables() doing a downward and overlapping move on a > mutually-aligned offset within a PMD. By mutual alignment, I > mean the source and destination addresses of the mremap are at > the same offset within a PMD. > > This mutual alignment along with the fact that the move is downward is > sufficient to cause a warning related to having an allocated PMD that > does not have PTEs in it. > > This warning will only trigger when there is mutual alignment in the > move operation. A solution, as suggested by Linus Torvalds [2], is to > initiate the copy process at the PMD level whenever such alignment is > present. Implementing this approach will not only prevent the warning > from being triggered, but it will also optimize the operation as this > method should enhance the speed of the copy process whenever there's a > possibility to start copying at the PMD level. > > Some more points: > a. The optimization can be done only when both the source and > destination of the mremap do not have anything mapped below it up to a > PMD boundary. I add support to detect that. > > b. #1 is not a problem for the call to move_page_tables() from exec.c as > nothing is expected to be mapped below the source. However, for > non-overlapping mutually aligned moves as triggered by mremap(2), I > added support for checking such cases. > > c. I currently only optimize for PMD moves, in the future I/we can build > on this work and do PUD moves as well if there is a need for this. But I > want to take it one step at a time. > > d. We need to be careful about mremap of ranges within the VMA itself. > For this purpose, I added checks to determine if the address after > alignment falls within its VMA itself. > > [1] https://lore.kernel.org/all/ZB2GTBD%2FLWTrkOiO@dhcp22.suse.cz/ > [2] https://lore.kernel.org/all/CAHk-=whd7msp8reJPfeGNyt0LiySMT0egExx3TVZSX3Ok6X=9g@mail.gmail.com/ > > Suggested-by: Linus Torvalds > Signed-off-by: Joel Fernandes (Google) > --- > mm/mremap.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 62 insertions(+) > > diff --git a/mm/mremap.c b/mm/mremap.c > index 11e06e4ab33b..035fbf542a8f 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -489,6 +489,53 @@ static bool move_pgt_entry(enum pgt_entry entry, struct vm_area_struct *vma, > return moved; > } > > +/* > + * A helper to check if a previous mapping exists. Required for > + * move_page_tables() and realign_addr() to determine if a previous mapping > + * exists before we can do realignment optimizations. > + */ > +static bool can_align_down(struct vm_area_struct *vma, unsigned long addr_to_align, > + unsigned long mask) > +{ > + unsigned long addr_masked = addr_to_align & mask; > + > + /* > + * If @addr_to_align of either source or destination is not the beginning > + * of the corresponding VMA, we can't align down or we will destroy part > + * of the current mapping. > + */ > + if (vma->vm_start != addr_to_align) > + return false; > + > + /* > + * Make sure the realignment doesn't cause the address to fall on an > + * existing mapping. > + */ > + return find_vma_intersection(vma->vm_mm, addr_masked, addr_to_align) == NULL; > +} > + > +/* Opportunistically realign to specified boundary for faster copy. */ > +static void try_realign_addr(unsigned long *old_addr, struct vm_area_struct *old_vma, > + unsigned long *new_addr, struct vm_area_struct *new_vma, > + unsigned long mask) > +{ > + /* Skip if the addresses are already aligned. */ > + if ((*old_addr & ~mask) == 0) > + return; > + > + /* Only realign if the new and old addresses are mutually aligned. */ > + if ((*old_addr & ~mask) != (*new_addr & ~mask)) > + return; > + > + /* Ensure realignment doesn't cause overlap with existing mappings. */ > + if (!can_align_down(old_vma, *old_addr, mask) || > + !can_align_down(new_vma, *new_addr, mask)) > + return; > + > + *old_addr = *old_addr & mask; > + *new_addr = *new_addr & mask; > +} > + > unsigned long move_page_tables(struct vm_area_struct *vma, > unsigned long old_addr, struct vm_area_struct *new_vma, > unsigned long new_addr, unsigned long len, > @@ -508,6 +555,14 @@ unsigned long move_page_tables(struct vm_area_struct *vma, > return move_hugetlb_page_tables(vma, new_vma, old_addr, > new_addr, len); > > + /* > + * If possible, realign addresses to PMD boundary for faster copy. > + * Only realign if the mremap copying hits a PMD boundary. > + */ > + if ((vma != new_vma) > + && (len >= PMD_SIZE - (old_addr & ~PMD_MASK))) > + try_realign_addr(&old_addr, vma, &new_addr, new_vma, PMD_MASK); > + > flush_cache_range(vma, old_addr, old_end); > mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma->vm_mm, > old_addr, old_end); > @@ -577,6 +632,13 @@ unsigned long move_page_tables(struct vm_area_struct *vma, > > mmu_notifier_invalidate_range_end(&range); > > + /* > + * Prevent negative return values when {old,new}_addr was realigned > + * but we broke out of the above loop for the first PMD itself. > + */ > + if (len + old_addr < old_end) > + return 0; > + > return len + old_addr - old_end; /* how much done */ > } > > -- > 2.42.0.rc1.204.g551eb34607-goog > Looks good to me! Thanks for the changes :) Reviewed-by: Lorenzo Stoakes