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Wed, 2 Sep 2026 12:25:58 +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 uZYNENYVmGplFwAAD6G6ig (envelope-from ); Wed, 02 Sep 2026 12:25:58 +0000 Date: Wed, 2 Sep 2026 13:25:56 +0100 From: Pedro Falcato To: Eli Billauer Cc: Mike Rapoport , David Laight , Arnd Bergmann , Brad Warrum , Greg Kroah-Hartman , Michal Simek , Ritu Agarwal , Andrew Morton , David Hildenbrand , Matthew Wilcox , Vlastimil Babka , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH 2/4] char: xillybus: replace __get_free_pages() with kmalloc() Message-ID: References: <20260830-char-misc-v1-0-05e2ce44f291@kernel.org> <20260830-char-misc-v1-2-05e2ce44f291@kernel.org> <20260831123937.32255a49@pumpkin> <1729fb06-6beb-f8ef-3e71-47fc3f00345d@outbound.gmail.com> <9a3d6a08-339c-3315-cc81-46907d0836ad@outbound.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <9a3d6a08-339c-3315-cc81-46907d0836ad@outbound.gmail.com> X-Rspamd-Action: no action X-Rspam-User: X-Stat-Signature: wks7iia851k37w9x94unm1uz4gc533qd X-Rspamd-Queue-Id: 2931A8000F X-Rspamd-Server: rspam06 X-HE-Tag: 1788351969-717321 X-HE-Meta: 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 8PPiIZZp dL+wFv64qbzKpx/P5acj9k9vnO8kk0UrP4m7/L+i0NxcGV3vCHF+hEdtiasoAlxwbLRDgXHxvtc2W0t15VSD5Zt9NeiTKycxa90HIAKkqRfkqiJ57sR0HgAfL9kN3jBnk/B+nJjF2bjtOR+daJ9mme/jN9M+CgXZCfZKqSIFo5IuLnmghzL1+ppSz9boavGHr+MzHJv/FeOOlFsyYPJJfDzcag6P6QM45w/gNTkuIE76gmw9mfdRGhj6UWnkiP+z0SdW8CbBgkD20lOO+3vMiH5T+cWhW9kbo+K3r+RTvnYco4e7R7unNklaps92kg7uvL6g49tnkybI37c6lnP2XUNi6wDwF+Buy3wotbOzni8ZC7KtoMQ/kYWgn5tOWQUvW1QxUIFkh3cQbuRXJ9QNMBooyeB7iVf7MaKzbRpOeEUTaJRE= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, Sep 02, 2026 at 01:41:54PM +0200, Eli Billauer wrote: > On 01/09/2026 19:46, Mike Rapoport wrote: > > > * Does vmalloc() guarantee that non-pageable physical RAM is allocated when > > > it returns? > > It's not pageable in the sense of demand paging. Some architectures lazily > > synchronize vmalloc page tables and this can cause page faults that will > > take care of the page table synchronization. > > > > Thanks for this clarification. > > This sounds like a showstopper to me. Immediately after the allocation of > these buffers, data from the USB device can arrive at 400 MB/s. If the data > flow is throttled as a result of handling page faults while trying to write Note that these page faults are supposed to be uncommon in nature. They will only happen in architectures where the top-level page tables are not pre-populated, and when they weren't populated before. On a 64-bit architecture that would be something like every 512 GiB of address space :) > the data to these buffers, this could lead to an overflow in the USB > device's own RAM buffers. This is because the USB device is an FPGA, where > RAM is an expensive resource. > > So using vmalloc() may result in a malfunction in the data transport where > the __get_free_pages() would have been successful. This outweighs the > benefit of a somewhat higher probability of success in allocating very large > buffers, and surely the improved aesthetics of the kernel code. > > Regards, > Eli > -- Pedro