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Mon, 22 Dec 2025 21:30:02 -0800 (PST) From: Askar Safin To: gmazyland@gmail.com Cc: Dell.Client.Kernel@dell.com, dm-devel@lists.linux.dev, linux-block@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-crypto@vger.kernel.org, linux-lvm@lists.linux.dev, linux-mm@kvack.org, linux-pm@vger.kernel.org, linux-raid@vger.kernel.org, lvm-devel@lists.linux.dev, mpatocka@redhat.com, pavel@ucw.cz, rafael@kernel.org Subject: Re: [RFC PATCH 2/2] swsusp: make it possible to hibernate to device mapper devices Date: Tue, 23 Dec 2025 08:29:52 +0300 Message-ID: <20251223052952.2623971-1-safinaskar@gmail.com> X-Mailer: git-send-email 2.47.3 In-Reply-To: <86300955-72e4-42d5-892d-f49bdf14441e@gmail.com> References: <86300955-72e4-42d5-892d-f49bdf14441e@gmail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Queue-Id: 31B15C0002 X-Stat-Signature: qzoz8y5ocnpbdxdy6jkf1b486spfsj4c X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1766467805-838714 X-HE-Meta: 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 KvJGj4xg 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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: Milan Broz : > Anyway, my understanding is that all device-mapper targets use mempools, > which should ensure that they can process even under memory pressure. I used journal mode so far, but, as well as I understand, direct mode is okay for my use case. Okay, I spent some time carefully reading dm-integrity source code. I have read v6.12.48, because this is kernel I use. And I conclude that dm-integrity code never allocate (not even from mempool)... ...in main code paths (as opposed to initialization code paths)... ...in direct ('D') mode... ...if I/O doesn't fail and checksums match. (As I said in previous letter, mempools are bad, too, as well as I understand.) I found exactly one place, where we seem to allocate in main code path: https://elixir.bootlin.com/linux/v6.12.48/source/drivers/md/dm-integrity.c#L1789 (i. e. these two kmalloc's). But I think this okay, because: - we pass GFP_NOIO, so, as well as I understand, this should not lead to recursion - we pass __GFP_NORETRY, so, as well as I understand, we will not block in this kmalloc for too much time - we gracefully handle possible failure Other strange place I found is this: https://elixir.bootlin.com/linux/v6.12.48/source/drivers/md/dm-integrity.c#L1704 . But I think this is okay, because: - integrity_recheck is only ever called from here: https://elixir.bootlin.com/linux/v6.12.48/source/drivers/md/dm-integrity.c#L1857 - that integrity_recheck call is only ever happens if dm_integrity_rw_tag failed - as well as I understand, dm_integrity_rw_tag can only fail if we got actual I/O error or checksum mismatch So, this mempool_alloc call is okay for my use case. So: in 'D' mode everything should be okay for my use case. Another note: I used very stupid way to search functions, which allocate: if function has "alloc" in its name, then I consider it allocating. :) And final note: there is an elephant in a room: bufio. As well as I understand, when pages are swapped in my use case, they first will get to dm-integrity bufio cache, and only after that, they will actually hit disk. This, of course, defeats whole purpose of swap. And possibly can lead to deadlocks. Is there a way to disable bufio? Or maybe bufio is used for checksums and metadata only? -- Askar Safin