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Wed, 23 Oct 2024 15:01:29 +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 cHh7KskPGWdTIAAAD6G6ig (envelope-from ); Wed, 23 Oct 2024 15:01:29 +0000 Message-ID: Date: Wed, 23 Oct 2024 17:01:29 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH hotfix 6.12 8/8] mm: do not attempt second merge for file-backed VMAs Content-Language: en-US To: Lorenzo Stoakes , Andrew Morton Cc: "Liam R . Howlett" , Jann Horn , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Linus Torvalds , Peter Xu References: <7b6bf6165080505feb5e00b313d2c84c25015e45.1729628198.git.lorenzo.stoakes@oracle.com> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; keydata= xsFNBFZdmxYBEADsw/SiUSjB0dM+vSh95UkgcHjzEVBlby/Fg+g42O7LAEkCYXi/vvq31JTB KxRWDHX0R2tgpFDXHnzZcQywawu8eSq0LxzxFNYMvtB7sV1pxYwej2qx9B75qW2plBs+7+YB 87tMFA+u+L4Z5xAzIimfLD5EKC56kJ1CsXlM8S/LHcmdD9Ctkn3trYDNnat0eoAcfPIP2OZ+ 9oe9IF/R28zmh0ifLXyJQQz5ofdj4bPf8ecEW0rhcqHfTD8k4yK0xxt3xW+6Exqp9n9bydiy tcSAw/TahjW6yrA+6JhSBv1v2tIm+itQc073zjSX8OFL51qQVzRFr7H2UQG33lw2QrvHRXqD Ot7ViKam7v0Ho9wEWiQOOZlHItOOXFphWb2yq3nzrKe45oWoSgkxKb97MVsQ+q2SYjJRBBH4 8qKhphADYxkIP6yut/eaj9ImvRUZZRi0DTc8xfnvHGTjKbJzC2xpFcY0DQbZzuwsIZ8OPJCc LM4S7mT25NE5kUTG/TKQCk922vRdGVMoLA7dIQrgXnRXtyT61sg8PG4wcfOnuWf8577aXP1x 6mzw3/jh3F+oSBHb/GcLC7mvWreJifUL2gEdssGfXhGWBo6zLS3qhgtwjay0Jl+kza1lo+Cv BB2T79D4WGdDuVa4eOrQ02TxqGN7G0Biz5ZLRSFzQSQwLn8fbwARAQABzSBWbGFzdGltaWwg QmFia2EgPHZiYWJrYUBzdXNlLmN6PsLBlAQTAQoAPgIbAwULCQgHAwUVCgkICwUWAgMBAAIe AQIXgBYhBKlA1DSZLC6OmRA9UCJPp+fMgqZkBQJkBREIBQkRadznAAoJECJPp+fMgqZkNxIQ ALZRqwdUGzqL2aeSavbum/VF/+td+nZfuH0xeWiO2w8mG0+nPd5j9ujYeHcUP1edE7uQrjOC Gs9sm8+W1xYnbClMJTsXiAV88D2btFUdU1mCXURAL9wWZ8Jsmz5ZH2V6AUszvNezsS/VIT87 AmTtj31TLDGwdxaZTSYLwAOOOtyqafOEq+gJB30RxTRE3h3G1zpO7OM9K6ysLdAlwAGYWgJJ V4JqGsQ/lyEtxxFpUCjb5Pztp7cQxhlkil0oBYHkudiG8j1U3DG8iC6rnB4yJaLphKx57NuQ PIY0Bccg+r9gIQ4XeSK2PQhdXdy3UWBr913ZQ9AI2usid3s5vabo4iBvpJNFLgUmxFnr73SJ KsRh/2OBsg1XXF/wRQGBO9vRuJUAbnaIVcmGOUogdBVS9Sun/Sy4GNA++KtFZK95U7J417/J Hub2xV6Ehc7UGW6fIvIQmzJ3zaTEfuriU1P8ayfddrAgZb25JnOW7L1zdYL8rXiezOyYZ8Fm ZyXjzWdO0RpxcUEp6GsJr11Bc4F3aae9OZtwtLL/jxc7y6pUugB00PodgnQ6CMcfR/HjXlae h2VS3zl9+tQWHu6s1R58t5BuMS2FNA58wU/IazImc/ZQA+slDBfhRDGYlExjg19UXWe/gMcl De3P1kxYPgZdGE2eZpRLIbt+rYnqQKy8UxlszsBNBFsZNTUBCACfQfpSsWJZyi+SHoRdVyX5 J6rI7okc4+b571a7RXD5UhS9dlVRVVAtrU9ANSLqPTQKGVxHrqD39XSw8hxK61pw8p90pg4G /N3iuWEvyt+t0SxDDkClnGsDyRhlUyEWYFEoBrrCizbmahOUwqkJbNMfzj5Y7n7OIJOxNRkB IBOjPdF26dMP69BwePQao1M8Acrrex9sAHYjQGyVmReRjVEtv9iG4DoTsnIR3amKVk6si4Ea X/mrapJqSCcBUVYUFH8M7bsm4CSxier5ofy8jTEa/CfvkqpKThTMCQPNZKY7hke5qEq1CBk2 wxhX48ZrJEFf1v3NuV3OimgsF2odzieNABEBAAHCwXwEGAEKACYCGwwWIQSpQNQ0mSwujpkQ PVAiT6fnzIKmZAUCZAUSmwUJDK5EZgAKCRAiT6fnzIKmZOJGEACOKABgo9wJXsbWhGWYO7mD 8R8mUyJHqbvaz+yTLnvRwfe/VwafFfDMx5GYVYzMY9TWpA8psFTKTUIIQmx2scYsRBUwm5VI EurRWKqENcDRjyo+ol59j0FViYysjQQeobXBDDE31t5SBg++veI6tXfpco/UiKEsDswL1WAr tEAZaruo7254TyH+gydURl2wJuzo/aZ7Y7PpqaODbYv727Dvm5eX64HCyyAH0s6sOCyGF5/p eIhrOn24oBf67KtdAN3H9JoFNUVTYJc1VJU3R1JtVdgwEdr+NEciEfYl0O19VpLE/PZxP4wX PWnhf5WjdoNI1Xec+RcJ5p/pSel0jnvBX8L2cmniYnmI883NhtGZsEWj++wyKiS4NranDFlA HdDM3b4lUth1pTtABKQ1YuTvehj7EfoWD3bv9kuGZGPrAeFNiHPdOT7DaXKeHpW9homgtBxj 8aX/UkSvEGJKUEbFL9cVa5tzyialGkSiZJNkWgeHe+jEcfRT6pJZOJidSCdzvJpbdJmm+eED w9XOLH1IIWh7RURU7G1iOfEfmImFeC3cbbS73LQEFGe1urxvIH5K/7vX+FkNcr9ujwWuPE9b 1C2o4i/yZPLXIVy387EjA6GZMqvQUFuSTs/GeBcv0NjIQi8867H3uLjz+mQy63fAitsDwLmR EP+ylKVEKb0Q2A== In-Reply-To: <7b6bf6165080505feb5e00b313d2c84c25015e45.1729628198.git.lorenzo.stoakes@oracle.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Stat-Signature: mirzhtompir845ie45qxdwryeg7za89e X-Rspamd-Queue-Id: 7842840016 X-Rspamd-Server: rspam02 X-HE-Tag: 1729695679-383381 X-HE-Meta: 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 rjL8+Q8o ugRt8DlKgW33GutFEfJV9eY5G3O2zCEMQ975EdH9QmZrkVj0G7IrmFnyOf4E3iipFS+gF/hguZwmy5eB7zurCJ+Dqk7MuSB2Aj7cGlzfwdNlpHyMLLZQ/0vFnoSj7SRuqMr1csSID65WJ4WsxFdAk6aj7JwdcD9m2bsXCcob93geUgBcZmcvg+s+0k3F0S3Qd1cXa4i8WJhPv1TD/RQ3t1qmFddetjAn10ka7CzqQDb6s6XAhC5udaXxe9j/BjXYKMx0xKyw05ON53MUmoBn77tzxeT5PYxEZwBOs0ipDZaWTYCbUSdva6xkUv3s7R3YP1ApxLQnGqC34xsX3/sHptkzvnff7ofzJK7Ga5oKNUdrBRfCtIafonOZEygYJLUU2efCAX4ckE+SY8c8cLryr/iyjuw== 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 10/22/24 22:40, Lorenzo Stoakes wrote: > Previously, we'd always try to merge a file-backed VMA if its flags were > changed by the driver. > > This however is rarely meaningful as typically the flags would be changed > to VM_PFNMAP or other VM_SPECIAL flags which are inherently unmergable. > > In cases where it is meaningful (for instance DAX) it is doubtful that this Hm if that's true, I'm imagining many piecemeal mmap()s of DAX that used to merge but now will create tons of VMA's, which doesn't sound great. Then it has also potentially breaking effects on mremap() which doesn't work accross multiple VMA's. > optimisation is worth the effort and maintenance risk of having to unwind > state and perform a merge. What if we simply created a new vma but only then checked if the flags changed and we can merge it with neighbours (i.e. like the mprotect() merging case). Less efficient, but less tricky and with the same result hopefully? > Since we've observed bugs and resource leaks due to complexity in this > area, it is simply not acceptable to have a 'nice to have' optimisation > like this complicating an already very complicated code path, so let's > simply eliminate it. > > Signed-off-by: Lorenzo Stoakes > --- > mm/vma.c | 39 ++------------------------------------- > 1 file changed, 2 insertions(+), 37 deletions(-) > > diff --git a/mm/vma.c b/mm/vma.c > index a271e2b406ab..fe1fe5099e78 100644 > --- a/mm/vma.c > +++ b/mm/vma.c > @@ -2260,8 +2260,7 @@ static int __mmap_prepare(struct mmap_state *map) > return 0; > } > > -static int __mmap_new_file_vma(struct mmap_state *map, struct vm_area_struct *vma, > - struct vm_area_struct **mergep) > +static int __mmap_new_file_vma(struct mmap_state *map, struct vm_area_struct *vma) > { > struct vma_iterator *vmi = map->vmi; > struct vma_merge_struct *vmg = map->vmg; > @@ -2291,34 +2290,6 @@ static int __mmap_new_file_vma(struct mmap_state *map, struct vm_area_struct *vm > (vma->vm_flags & VM_MAYWRITE)); > > vma_iter_config(vmi, vmg->start, vmg->end); > - /* > - * If flags changed after mmap_file(), we should try merge > - * vma again as we may succeed this time. > - */ > - if (unlikely(map->flags != vma->vm_flags && vmg->prev)) { > - struct vm_area_struct *merge; > - > - vmg->flags = vma->vm_flags; > - /* If this fails, state is reset ready for a reattempt. */ > - merge = vma_merge_new_range(vmg); > - > - if (merge) { > - /* > - * ->mmap() can change vma->vm_file and fput > - * the original file. So fput the vma->vm_file > - * here or we would add an extra fput for file > - * and cause general protection fault > - * ultimately. > - */ > - fput(vma->vm_file); > - vm_area_free(vma); > - vma_iter_free(vmi); > - *mergep = merge; > - } else { > - vma_iter_config(vmi, vmg->start, vmg->end); > - } > - } > - > map->flags = vma->vm_flags; > return 0; > } > @@ -2341,7 +2312,6 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap) > { > struct vma_iterator *vmi = map->vmi; > struct vma_merge_struct *vmg = map->vmg; > - struct vm_area_struct *merge = NULL; > int error = 0; > struct vm_area_struct *vma; > > @@ -2365,7 +2335,7 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap) > } > > if (vmg->file) > - error = __mmap_new_file_vma(map, vma, &merge); > + error = __mmap_new_file_vma(map, vma); > else if (map->flags & VM_SHARED) > error = shmem_zero_setup(vma); > else > @@ -2374,9 +2344,6 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap) > if (error) > goto free_iter_vma; > > - if (merge) > - goto file_expanded; > - > #ifdef CONFIG_SPARC64 > /* TODO: Fix SPARC ADI! */ > WARN_ON_ONCE(!arch_validate_flags(map->flags)); > @@ -2393,8 +2360,6 @@ static int __mmap_new_vma(struct mmap_state *map, struct vm_area_struct **vmap) > * call covers the non-merge case. > */ > khugepaged_enter_vma(vma, map->flags); > - > -file_expanded: > ksm_add_vma(vma); > > *vmap = vma; > -- > 2.47.0