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Thu, 1 Aug 2024 08:14:28 +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 zKNFAORDq2bzDgAAD6G6ig (envelope-from ); Thu, 01 Aug 2024 08:14:28 +0000 Date: Thu, 1 Aug 2024 10:14:26 +0200 From: Oscar Salvador To: Lorenzo Stoakes Cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Peter Xu , Muchun Song , David Hildenbrand , Donet Tom , Matthew Wilcox , Vlastimil Babka , Michal Hocko Subject: Re: [PATCH v2 6/9] mm: Make hugetlb mappings go through mm_get_unmapped_area_vmflags Message-ID: References: <20240729091018.2152-1-osalvador@suse.de> <20240729091018.2152-7-osalvador@suse.de> <8a57e184-4994-4642-959d-44dc7efbceca@lucifer.local> <354e4db4-e257-48a4-9e05-7f0595728ec6@lucifer.local> <47de87e4-47df-4024-a313-03cc14b8e248@lucifer.local> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <47de87e4-47df-4024-a313-03cc14b8e248@lucifer.local> X-Rspamd-Action: no action X-Stat-Signature: i8jcufkcuoz46dty5j5hb4jj5o5zg8fs X-Rspam-User: X-Rspamd-Queue-Id: 56DC020021 X-Rspamd-Server: rspam02 X-HE-Tag: 1722500070-534950 X-HE-Meta: U2FsdGVkX19dIPxs4LOP44t4gAu2YGrf6Lq/388jdCC6UeqQsBj1i49rTQ0xj3UBgZDeethb3pBHCPpoSZFIEKAYxiQqZFawCrSz2OOQCv7xbXVmUs/FvrBPrFso26dKCe30UZeO5SZW/R/ropa83gcaeYTTE8aOeLJzX519svlZZsHFDZhThQZzpFhF/z34p/NkbjujcyUlCW6HlVP/FVqOdAnVgI3irdi/Le9scxY8fXBuQB/yKaRSS5WJS8W4RLqqaqcz1l6ti0UOTcCoLErV4EUJ8vbVfEU8AawTOkHMPr3BJ9W5PMvWuILhBfR06tEo4Ly0ladrEJI+m7cNU2tVTi/sI1MjfIXenrX+zd8cMgys+22Y7yZZdqTMg9185+zqLhfLrbrsdmnY1hJXEPSH360TIjaniPGt6QcEGUCVtKcZSJJ4Qs+1tAjgKnEvUP5lHxxGRkrI2SMqFfW0GpotZPlO0knNGlD+Q+LjyIjjIJPvYjhj9pVivJLgIKQNBQydP0ZFXtiDGGyZavlpKtn5kxadoS6pDZAnKfLrjTS5qANMdbrHKVFiTRuSAm0iLqRHCqy6Ea+vbylyxRQprrwgidst9ymKSN7mrSKb4JjkYf7SdUN+KlWH4KZozIaJp61l/rhauiNYQBjk2vR/mcT1LMHuj+DU7EhJBxLXqBM+mvo9/jx1R6gr0C5bayRs8BCAUeFOKZF+M+A6t8DOs32aZR6m/1BZDnOMGSBAkJdiMhmr4iCmdA/0i4ZCOusFc1AziKiMnRv3Q7+4n6zMvDzNObdVT0fQ/yNu7NlnBpDkd3Yik/Lp1p0pQp4gydHYE6ukxxrcG4kaPJC8GIg0Ckn2kG2nAarutz6+bje5sYN6SR5yftvll/fuiHVPmShe91sXX/4Y1pLCZYSJwVBfsbz9CAsAFHevJOjdXq40w9d2oiWBoBKhSHlmfsQ52derFm/Lly/B0AKyZPtY4oz abTuBWEt /G0cjc1xWY/gN5xgxOnC+Npd9KEkTfKZqpZpM+am1MnAJzN7AXALeeWFAa0yAGqGjvzIDZiq9kZngsdXpcg9JYJXrrASZrjl7wooQkzZebWiEwLEPK906Coe+JrfbJ4PVmgqpj0VHiKEnXZexCswqPTfYbzWiIJgikFgrM0gm0Utkk+sJ0/Cn3zFTYiy+LaFKTDORYwRAJWHJ0EGF1X/7F6ITXv+g1VEfv1MeRlVY3fOg2RZrHBCz19eU8NOq2oaWpb5MUJCjQUAKpn1N8l7PNQb+PX/7j5yb1B7X 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 Wed, Jul 31, 2024 at 05:15:41PM +0100, Lorenzo Stoakes wrote: > I've not got the vm debug on in my build, so it's blowing up here for me: > > static unsigned long shm_get_unmapped_area(struct file *file, > unsigned long addr, unsigned long len, unsigned long pgoff, > unsigned long flags) > { > struct shm_file_data *sfd = shm_file_data(file); > > return sfd->file->f_op->get_unmapped_area(sfd->file, addr, len, > pgoff, flags); > } > > Notice that that doesn't check whether sfd->file->f_op->get_unmapped_area > is NULL. I see now, thanks. > So since you remove this from the f_ops, it causes a NULL pointer deref. ... > static const struct file_operations shm_file_operations = { > .. > .get_unmapped_area = shm_get_unmapped_area, > ... > }; > > Then this get_area() is invoked, which calls shm_get_unmapped_area(), which > calls f_op->get_unmapped_area() on your hugetlbfs_file_operations object > which you just deleted and it's NULL. > > This is why you have to be super careful here, there's clearly stuff out > there that assumes that this can't happen, which you need to track down. > > A quick grep however _suggests_ this might be the one landmine place. But > you need to find a smart way to deal with this. Probably, the most straightforward way to fix this is to instead of setting .get_unmapped_area to NULL for hugetlbfs_file_operations, would be to have it re-defined like: .get_unmapped_area = mm_get_unmapped_area_vmflags Which is what we call after this patchset. So no more things have to tweaked. On a more correct way, __maybe__ have something like: diff --git a/ipc/shm.c b/ipc/shm.c index 3e3071252dac..222dca8a3716 100644 --- a/ipc/shm.c +++ b/ipc/shm.c @@ -648,8 +648,11 @@ static unsigned long shm_get_unmapped_area(struct file *file, { struct shm_file_data *sfd = shm_file_data(file); - return sfd->file->f_op->get_unmapped_area(sfd->file, addr, len, + if (sfd->file->f_op->get_unmapped_area) + return sfd->file->f_op->get_unmapped_area(sfd->file, addr, len, pgoff, flags); + + return mm_get_unmapped_area_vmflags(sfd->file, addr, len, pgoff, flags); } static const struct file_operations shm_file_operations = { Still unsure about which approach looks more correct though. -- Oscar Salvador SUSE Labs