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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: List-Subscribe: List-Unsubscribe: On Tue, Jul 01, 2025 at 12:00:21PM +0530, Dev Jain wrote: > > On 01/07/25 11:23 am, Lorenzo Stoakes wrote: > > On Tue, Jul 01, 2025 at 11:15:25AM +0530, Dev Jain wrote: > > > Sorry I am not following, don't know in detail about the VMA merge stuff. > > > Are you saying the after the patch, the VMAs will eventually get merged? > > > Is it possible in the kernel to get a merge in the "future"; as I understand > > > it only happens at mmap() time? > > > > > > Suppose before the patch, you have two consecutive VMAs between (PMD, 2*PMD) size. > > > If they are able to get merged after the patch, why won't they be merged before the patch, > > > since the VMA characteristics are the same? > > > > > > > > Rik's patch aligned each to 2 MiB boundary. So you'd get gaps: > > > > > > 0 2MB 4MB 6MB 8MB 10MB > > |-------------.------| |-------------.------| |-------------.------| > > | . | | . | | . | > > | . | | . | | . | > > |-------------.------| |-------------.------| |-------------.------| > > huge mapped 4k m'd > > The effort to draw this is appreciated! > > I understood the alignment, what I am asking is this: > > In __get_unmapped_area(), we will return a THP-aligned addr from > thp_get_unmapped_area_vmflags(). Now for the diagram you have > drawn, suppose that before the patch, we first mmap() the > 8MB-start chunk. Then we mmap the 4MB start chunk. > We go to __mmap_region(), and we see that the 8MB-start chunk > has mergeable characteristics, so we merge. So the gap goes away? No because there's a gap, we only merge immedaitely adjacent VMAs. And obviously gaps mean page tables wouldn't be adjacent either... The get_unmmaped_area() would have otherwise given adjacent mappings. Vlasta's patch means in this case we no longer bother trying to align these because their _length_ isn't PMD aligned.