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Wed, 20 May 2026 17:00:37 +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 WQvLALXoDWpmDQAAD6G6ig (envelope-from ); Wed, 20 May 2026 17:00:37 +0000 Date: Wed, 20 May 2026 19:00:31 +0200 From: Pedro Falcato To: Daniel Palmer Cc: akpm@linux-foundation.org, liam@infradead.org, ljs@kernel.org, vbabka@kernel.org, jannh@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH] mm/nommu: Implement just enough vmap that compressed erofs can be mounted Message-ID: References: <20260520163442.1099667-1-daniel@thingy.jp> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260520163442.1099667-1-daniel@thingy.jp> X-Spam-Flag: NO X-Spam-Score: -3.80 X-Spamd-Result: default: False [-3.80 / 50.00]; BAYES_HAM(-3.00)[99.99%]; NEURAL_HAM_LONG(-1.00)[-1.000]; MID_RHS_NOT_FQDN(0.50)[]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RCVD_VIA_SMTP_AUTH(0.00)[]; MISSING_XM_UA(0.00)[]; MIME_TRACE(0.00)[0:+]; ARC_NA(0.00)[]; RCPT_COUNT_SEVEN(0.00)[8]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[thingy.jp:email] X-Spam-Level: Hello, On Thu, May 21, 2026 at 01:34:38AM +0900, Daniel Palmer wrote: > This implements a very poor imitation of vmap that works just > enough that compressed erofs filesystems can be mounted on nommu > machines. Right now compressed erofs filesystems trigger a BUG() > on nommu due to this missing. > > This is awful, doesn't work like real vmap etc,.. but if you > really cared about stuff working you'd have an MMU I guess? > > Signed-off-by: Daniel Palmer > --- > > Did I miss anything massive that is going to come back and bite me? > Maybe it would have made more sense just to change the erofs > code so on !CONFIG_MMU it doesn't use vmap? Yes, I think that would be preferrable. I'm sure[1] it's possible to get EROFS to not use vmap(). I know your patch took some effort but trying to implement this for nommu is fundamentally not possible. Like your vmap() "bouncing" is completely fake and users expecting that the pages are actually shared will have a nasty surprise. So, yeah, please see if you can find a way forward for erofs :) [1] I hope. -- Pedro