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Mon, 13 Jan 2025 11:02:32 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id SHIcB8jyhGcKdQAAD6G6ig (envelope-from ); Mon, 13 Jan 2025 11:02:32 +0000 Date: Mon, 13 Jan 2025 12:02:26 +0100 From: Oscar Salvador To: Peter Xu Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Breno Leitao , Rik van Riel , Muchun Song , Naoya Horiguchi , Roman Gushchin , Ackerley Tng , Andrew Morton , linux-stable Subject: Re: [PATCH v2 1/7] mm/hugetlb: Fix avoid_reserve to allow taking folio from subpool Message-ID: References: <20250107204002.2683356-1-peterx@redhat.com> <20250107204002.2683356-2-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250107204002.2683356-2-peterx@redhat.com> X-Rspamd-Action: no action X-Stat-Signature: csbcje5d4w7un3sqaxwbff7yb3j9on6m X-Rspamd-Queue-Id: 79DFAA000B X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1736766155-374251 X-HE-Meta: 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 mmhnVFqI v2kejf6DP7OKppQ8AG9iL1BV/WotF+SIxwPog1/QwZa0MJpqKiN5bMA5BenrLMWmNximBj6E2mAbGjI+hFI/yXrU4E0gVBpcui9e2JHR8mIYEvWFhzAZ4qu79FoLqq8kMTH8plxpH+PE9MBYEHE4j5NrN+kwubteyEd0wAEq2ti74Oa1O1KnzeZPuapgk7zlWYuCN6Xpib2dE8YRt1DgnX/HE7vI79fJOdGhdGzTc3BvaWmdwGfQfwVIqwOz4ovJX4Xylya/IKP0iyDklcUR8fGLwdqPL5ocfAh15MjV9rBIyQ3wvqUdcpaZhLnukPIDshl/L/cM8qLuqF7kwc+NHd2CQWukYUiqNcFW641kmJ9u+BUJPl1MqLU4XDmuzVepJMPnorV7CZ13eQlsRKasrwP/5b8LHOTLC2hEnhczz/akEWirXWgIj2K8p1sEhO3xFxuuy9jDMtAhqLwk= 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, Jan 07, 2025 at 03:39:56PM -0500, Peter Xu wrote: > Since commit 04f2cbe35699 ("hugetlb: guarantee that COW faults for a > process that called mmap(MAP_PRIVATE) on hugetlbfs will succeed"), > avoid_reserve was introduced for a special case of CoW on hugetlb private > mappings, and only if the owner VMA is trying to allocate yet another > hugetlb folio that is not reserved within the private vma reserved map. > > Later on, in commit d85f69b0b533 ("mm/hugetlb: alloc_huge_page handle areas > hole punched by fallocate"), alloc_huge_page() enforced to not consume any > global reservation as long as avoid_reserve=true. This operation doesn't > look correct, because even if it will enforce the allocation to not use > global reservation at all, it will still try to take one reservation from > the spool (if the subpool existed). Then since the spool reserved pages > take from global reservation, it'll also take one reservation globally. > > Logically it can cause global reservation to go wrong. > > I wrote a reproducer below, trigger this special path, and every run of > such program will cause global reservation count to increment by one, until > it hits the number of free pages: > > #define _GNU_SOURCE /* See feature_test_macros(7) */ > #include > #include > #include > #include > #include > #include > > #define MSIZE (2UL << 20) > > int main(int argc, char *argv[]) > { > const char *path; > int *buf; > int fd, ret; > pid_t child; > > if (argc < 2) { > printf("usage: %s \n", argv[0]); > return -1; > } > > path = argv[1]; > > fd = open(path, O_RDWR | O_CREAT, 0666); > if (fd < 0) { > perror("open failed"); > return -1; > } > > ret = fallocate(fd, 0, 0, MSIZE); > if (ret != 0) { > perror("fallocate"); > return -1; > } > > buf = mmap(NULL, MSIZE, PROT_READ|PROT_WRITE, > MAP_PRIVATE, fd, 0); > > if (buf == MAP_FAILED) { > perror("mmap() failed"); > return -1; > } > > /* Allocate a page */ > *buf = 1; > > child = fork(); > if (child == 0) { > /* child doesn't need to do anything */ > exit(0); > } > > /* Trigger CoW from owner */ > *buf = 2; > > munmap(buf, MSIZE); > close(fd); > unlink(path); > > return 0; > } > > It can only reproduce with a sub-mount when there're reserved pages on the > spool, like: > > # sysctl vm.nr_hugepages=128 > # mkdir ./hugetlb-pool > # mount -t hugetlbfs -o min_size=8M,pagesize=2M none ./hugetlb-pool > > Then run the reproducer on the mountpoint: > > # ./reproducer ./hugetlb-pool/test > > Fix it by taking the reservation from spool if available. In general, > avoid_reserve is IMHO more about "avoid vma resv map", not spool's. > > I copied stable, however I have no intention for backporting if it's not a > clean cherry-pick, because private hugetlb mapping, and then fork() on top > is too rare to hit. > > Cc: linux-stable > Fixes: d85f69b0b533 ("mm/hugetlb: alloc_huge_page handle areas hole punched by fallocate") > Reviewed-by: Ackerley Tng > Tested-by: Ackerley Tng > Signed-off-by: Peter Xu Reviewed-by: Oscar Salvador -- Oscar Salvador SUSE Labs