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Howlett" , "Paul E. McKenney" , Suren Baghdasaryan , Kalesh Singh , Lokesh Gidra Subject: Re: [PATCH v6 1/7] mm/mremap: Optimize the start addresses in move_page_tables() Message-ID: References: <20230903151328.2981432-1-joel@joelfernandes.org> <20230903151328.2981432-2-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230903151328.2981432-2-joel@joelfernandes.org> X-Rspamd-Queue-Id: C0D2C180017 X-Rspam-User: X-Rspamd-Server: rspam11 X-Stat-Signature: shxhg8y7r34rn8fex1ctni3xcedaw3yq X-HE-Tag: 1694178467-512410 X-HE-Meta: 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 SKjElob1 Letg+YD0T4AhczjLu+BQgq7wfBxDLzlWBLuaPaC2zlz2POHF5EdW6FRSFkrlBSUARUviz9LRkqQcXy0Ft3m6NyySSKPXidK1KKU8fJ5tcEpysUPHkboSY/UEo8GtXJSBeOfycllCzsPGzhQrv4gIt8wSgjGF3jdMHjjayAyKrcxMbyjm1j14k+jhQ48PNlFC9c+WUXgVcriATA1xkFRLSuYxaq5vwKfHakbo4V0SvcoatJhJCfOotWznW/qTzvQjgnaD46dT1F8no1qnmR9BKYBjr0ia5VHccKaanIFQPri+RFv4WeKZpLUopfibHN43+TD8ugcWTv120BA8uxbQjFL6vTki6lXozmx3/1XAmX2P6YUXGPv7852iNCbXAT59FemEKeVmnhJn6BOI1WOMrp/+Shs1YqoyRiR1pCQBKhdpsDoQBgDIDBSTcxsp7yF3cYZ4l4oD6R9xNPcY= 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: Sorry for being silent most of the time in this patch series and thanks for pushing it forward. On Sun 03-09-23 15:13:22, Joel Fernandes 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/ > > Reviewed-by: Lorenzo Stoakes > Suggested-by: Linus Torvalds > Signed-off-by: Joel Fernandes (Google) The patch looks good to me. Acked-by: Michal Hocko > --- > mm/mremap.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 62 insertions(+) > > diff --git a/mm/mremap.c b/mm/mremap.c > index 11e06e4ab33b..1011326b7b80 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, vma->vm_start) == 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 */ > } > > -- > 2.42.0.283.g2d96d420d3-goog -- Michal Hocko SUSE Labs