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[79.242.62.93]) by smtp.gmail.com with ESMTPSA id o12sm5246234wms.15.2021.09.23.10.42.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 23 Sep 2021 10:42:56 -0700 (PDT) Subject: Re: [PATCH v1] mm/vmalloc: fix exact allocations with an alignment > 1 To: Uladzislau Rezki Cc: LKML , Ping Fang , Andrew Morton , Roman Gushchin , Michal Hocko , Oscar Salvador , Linux Memory Management List References: <20210908132727.16165-1-david@redhat.com> <20210916193403.GA1940@pc638.lan> <221e38c1-4b8a-8608-455a-6bde544adaf0@redhat.com> <20210921221337.GA60191@pc638.lan> <7f62d710-ca85-7d33-332a-25ff88b5452f@redhat.com> <20210922104141.GA27011@pc638.lan> From: David Hildenbrand Organization: Red Hat Message-ID: <437ff5c9-1b36-8ef7-1ce6-b3125e42de93@redhat.com> Date: Thu, 23 Sep 2021 19:42:55 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210922104141.GA27011@pc638.lan> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 22.09.21 12:41, Uladzislau Rezki wrote: > On Wed, Sep 22, 2021 at 10:34:55AM +0200, David Hildenbrand wrote: >>>> No, that's leaking implementation details to the caller. And no, increasing >>>> the range and eventually allocating something bigger (e.g., placing a huge >>>> page where it might not have been possible) is not acceptable for KASAN. >>>> >>>> If you're terribly unhappy with this patch, >>> Sorry to say but it simple does not make sense. >>> >> >> Let's agree to disagree. >> >> find_vmap_lowest_match() is imprecise now and that's an issue for exact >> allocations. We can either make it fully precise again (eventually degrading >> allocation performance) or just special-case exact allocations to fix the >> regression. >> >> I decided to go the easy path and do the latter; I do agree that making >> find_vmap_lowest_match() fully precise again might be preferred -- we could >> have other allocations failing right now although there are still suitable >> holes. >> >> I briefly thought about performing the search in find_vmap_lowest_match() >> twice. First, start the search without an extended range, and fallback to >> the extended range if that search fails. Unfortunately, I think that still >> won't make the function completely precise due to the way we might miss >> searching some suitable subtrees. >> >>>> >>>> please suggest something reasonable to fix exact allocations: >>>> a) Fixes the KASAN use case. >>>> b) Allows for automatic placement of huge pages for exact allocations. >>>> c) Doesn't leak implementation details into the caller. >>>> >>> I am looking at it. >> > I am testing this: > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c > index dcf23d16a308..cdf3bda6313d 100644 > --- a/mm/vmalloc.c > +++ b/mm/vmalloc.c > @@ -1161,18 +1161,14 @@ find_vmap_lowest_match(unsigned long size, > { > struct vmap_area *va; > struct rb_node *node; > - unsigned long length; > > /* Start from the root. */ > node = free_vmap_area_root.rb_node; > > - /* Adjust the search size for alignment overhead. */ > - length = size + align - 1; > - > while (node) { > va = rb_entry(node, struct vmap_area, rb_node); > > - if (get_subtree_max_size(node->rb_left) >= length && > + if (get_subtree_max_size(node->rb_left) >= size && > vstart < va->va_start) { > node = node->rb_left; > } else { > @@ -1182,9 +1178,9 @@ find_vmap_lowest_match(unsigned long size, > /* > * Does not make sense to go deeper towards the right > * sub-tree if it does not have a free block that is > - * equal or bigger to the requested search length. > + * equal or bigger to the requested search size. > */ > - if (get_subtree_max_size(node->rb_right) >= length) { > + if (get_subtree_max_size(node->rb_right) >= size) { > node = node->rb_right; > continue; > } > @@ -1192,16 +1188,30 @@ find_vmap_lowest_match(unsigned long size, > /* > * OK. We roll back and find the first right sub-tree, > * that will satisfy the search criteria. It can happen > - * only once due to "vstart" restriction. > + * due to "vstart" restriction or an alignment overhead. > */ > while ((node = rb_parent(node))) { > va = rb_entry(node, struct vmap_area, rb_node); > if (is_within_this_va(va, size, align, vstart)) > return va; > > - if (get_subtree_max_size(node->rb_right) >= length && > + if (get_subtree_max_size(node->rb_right) >= size && > vstart <= va->va_start) { > + /* > + * Shift the vstart forward, so we do not loop over same > + * sub-tree force and back. The aim is to continue tree > + * scanning toward higher addresses cutting off previous > + * ones. > + * > + * Please note we update vstart with parent's start address > + * adding "1" because we do not want to enter same sub-tree > + * one more time after it has already been inspected and no > + * suitable free block found there. > + */ > + vstart = va->va_start + 1; > node = node->rb_right; > + > + /* Scan a sub-tree rooted at "node". */ > break; > } > } > > > so it handles any alignment and is accurate when it comes to searching the most > lowest free block when user wants to allocate with a special alignment value. > > Could you please help and test the KASAN use case? Sure, I'll give it a spin tomorrow! Thanks! -- Thanks, David / dhildenb