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Mon, 28 Sep 2026 04:09:15 -0700 (PDT) From: Uladzislau Rezki X-Google-Original-From: Uladzislau Rezki Date: Mon, 28 Sep 2026 13:09:13 +0200 To: Hao Ge Cc: Suren Baghdasaryan , "David Hildenbrand (Arm)" , "Lorenzo Stoakes (ARM)" , Luis Chamberlain , Petr Pavlu , Daniel Gomez , Sami Tolvanen , Aaron Tomlin , Kent Overstreet , Andrew Morton , linux-modules@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Sashiko , stable@vger.kernel.org Subject: Re: [PATCH v10 2/6] alloc_tag: clean up the populate failure path Message-ID: References: <20260915070001.113559-1-hao.ge@linux.dev> <20260915070001.113559-3-hao.ge@linux.dev> <60642a7f-2949-428a-8290-40d69c989a77@linux.dev> <1dff448c-9cbe-4276-bca1-d91283b0c7b4@linux.dev> Precedence: bulk X-Mailing-List: linux-modules@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Thu, Sep 17, 2026 at 10:32:48AM +0800, Hao Ge wrote: > Hi Suren、David、Lorenzo > > On 2026/9/17 09:12, Hao Ge wrote: > > Hi Suren > > > > On 2026/9/17 03:07, Suren Baghdasaryan wrote: > >> On Wed, Sep 16, 2026 at 9:31 AM Suren Baghdasaryan wrote: > >>> > >>> On Tue, Sep 15, 2026 at 11:00 PM Hao Ge wrote: > >>>> > >>>> Hi Suren > >>>> > >>>> Thanks for you review. > >>>> > >>>> On 2026/9/16 05:09, Suren Baghdasaryan wrote: > >>>>> On Mon, Sep 14, 2026 at 11:59 PM Hao Ge wrote: > >>>>>> > >>>>>> The reservation is already stored in the maple tree when > >>>>>> vm_module_tags_populate() fails. A failed load never unloads the > >>>>>> module, so nothing releases the entry. Release it and roll > >>>>>> module_tags.size back. Without the rollback a concurrent load that > >>>>>> already passed needs_section_mem() can reuse the freed gap, skip > >>>>>> vm_module_tags_populate() and write to unmapped memory. > >>>>>> > >>>>>> vmap_pages_range() may have installed some PTEs before failing. A > >>>>>> retry to populate the same range would BUG on them, so undo them, > >>>>>> but only if vmap actually ran. > >>>>>> > >>>>>> Fixes: 4835f747d3ed ("alloc_tag: support for page allocation tag compression") > >>>>>> Fixes: 0f9b685626da ("alloc_tag: populate memory for module tags as needed") > >>>>> > >>>>> If a patch fixes two other patches, it likely needs to be split so > >>>>> that each part fixes one patch only. But read on please. I'm not sure > >>>>> if one of the fixes is correct. > >>>>> > >>>> > >>>> Kept them in one patch because they only make sense together. > >>>> > >>>> After a failed populate the reservation is released and the > >>>> size rolled back, so the next load takes the same gap and calls > >>>> vm_module_tags_populate() again, and that retry would trip over > >>>> the PTEs the failed vmap left behind. > >>>> > >>>> Without the rollback a load that already passed needs_section_mem() > >>>> just reuses the gap, skips populate and writes to unmapped memory. > >>>> So splitting them leaves each half broken. > >>> > >>> Yeah, that's why I think vmap_pages_range() cleanup should be a > >>> completely separate patch with vmap_pages_range() doing cleanup > >>> itself. > >>> > > > > I think I get where you're coming from now. > > Thanks for taking the time to explain. > > > >>>> > >>>>>> Reported-by: Sashiko > >>>>>> Cc: stable@vger.kernel.org > >>>>>> Signed-off-by: Hao Ge > >>>>>> --- > >>>>>> mm/alloc_tag.c | 10 ++++++++++ > >>>>>> 1 file changed, 10 insertions(+) > >>>>>> > >>>>>> diff --git a/mm/alloc_tag.c b/mm/alloc_tag.c > >>>>>> index 2070e682fe10..95ddf5b743d0 100644 > >>>>>> --- a/mm/alloc_tag.c > >>>>>> +++ b/mm/alloc_tag.c > >>>>>> @@ -804,6 +804,13 @@ static int vm_module_tags_populate(void) > >>>>>> next_page, PAGE_SHIFT) < 0) { > >>>>>> release_pages_arg arg = { .pages = next_page }; > >>>>>> > >>>>>> + /* > >>>>>> + * vmap_pages_range() only runs once all pages were > >>>>>> + * allocated, and it may have installed some mappings > >>>>>> + * before failing. Undo them. > >>>>> > >>>>> If vmap_pages_range() failed to map the range, why should we need to > >>>>> undo it? If it indeed leaves some partial mapping when failing then I > >>>>> would argue that vmap_pages_range() should be undoing these partial > >>>>> mappings itself before returning the error. > >>>>> > >>>> Yeah it does - none of the paths under vmap_pages_range() undo themselves on error. > >>> > >>> Hmm. I wonder if this behavior is intentional or requires a fix. The > >>> only possible reason I can think of is performance but I can't imagine > >>> a failure to map is a performance-critical case. > >>> > >>> +David Hildenbrand (Red Hat), +Lorenzo Stoakes (Oracle) what do you > >> > >> Huh, my gmail thinks you are still at your previous companies :) > >> > > > > I manually fixed it on the mailing list for this thread. > > > >>> guys think? Shouldn't vmap_pages_range() undo its possible partial > >>> mapping when it fails? > >>> > >>> > >>>> pcpu_map_pages does perform cleanup on its own as well. There may be other similar sites. > >>>> https://elixir.bootlin.com/linux/v7.3-rc3/source/mm/percpu-vm.c#L255 > >>>> so I did the same here. > >>>> > >>>> If we really want vmap_pages_range() to handle this cleanup internally, I believe that would > >>>> be a relatively large change, and we would need to consider many more cases. So I think we keep > >>>> the current behaviour for now. > >>> > >>> Yeah, I understand it would require a deeper cut but if that's the > >>> right way to handle this we might as well do that. Let's see what MM > >>> folks think. > >>> > > > > Sounds good to me. Let's wait and hear what the MM folks say. > > > > I just looked into the call sites for __vmap_pages_range. > And I found there's a bug in one vmalloc caller [1]. > When gfp_mask has __GFP_NOFAIL, the code keeps retrying > __vmap_pages_range. If there are leftover PTE mappings, > this can trigger a BUG(); [2] > https://elixir.bootlin.com/linux/v7.3-rc3/source/mm/vmalloc.c#L3946 [1] > https://elixir.bootlin.com/linux/v7.3-rc3/source/mm/vmalloc.c#L121 [2] > Indeed the below code is incomplete: /* * page tables allocations ignore external gfp mask, enforce it * by the scope API */ flags = memalloc_apply_gfp_scope(gfp_mask); do { ret = __vmap_pages_range(addr, addr + size, prot, area->pages, page_shift, nested_gfp); if (nofail && (ret < 0)) schedule_timeout_uninterruptible(1); } while (nofail && (ret < 0)); memalloc_restore_scope(flags); and it should be fixed. As the __vmap_pages_range() can actually fail and leave some installed mappings, we have to perform some cleanups on failure or on repeat(if nofail is true). Below code: + /* + * vmap_pages_range() only runs once all pages were + * allocated, and it may have installed some mappings + * before failing. Undo them. + */ + if (nr == more_pages) + vunmap_range(phys_end, phys_end + (nr << PAGE_SHIFT)); we can use but we need to check if partially mapped space is not an issue for the vunmap_range(). And it looks like it is not. @@ -3953,8 +3990,15 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask, do { ret = __vmap_pages_range(addr, addr + size, prot, area->pages, page_shift, nested_gfp); - if (nofail && (ret < 0)) - schedule_timeout_uninterruptible(1); + if (ret < 0) { + /* + * __vmap_pages_range() may have installed some mappings + * before failing. Undo them before returning or retrying. + */ + vunmap_range(addr, addr + size); + if (nofail) + schedule_timeout_uninterruptible(1); + } } while (nofail && (ret < 0)); memalloc_restore_scope(flags); -- Uladzislau Rezki