Re: [PATCH v2 3/5] mm/vmalloc: undo partial mappings inside the mapping functions

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Oct 20, 2026 at 09:27:14PM +0800, Hao Ge wrote:
> 
> On 10/20/26 18:03, Hao Ge wrote:
> > Hi Suren and Ulad
> > 
> > Sashiko reported several issues, and they all look real.
> > I have some ideas for fixing the first few;
> > of course, please correct me if I get anything wrong.
> > 
> > I'd like to discuss the last issue with you all.
> > 
> > 
> > On 2026/10/20 13:45, sashiko-bot@kernel.org wrote:
> > > Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider:
> > > - [High] mm/vmalloc: infinite loop in vunmap_pte_range() on unaligned error rollback
> > > - [High] mm/vmalloc: __vmap_pages_range_noflush() calls cond_resched() in atomic context
> > > - [Medium] mm/vmalloc: vmap_page_range() calls flush_cache_vmap() before error rollback
> > > --
> > > 
> > > commit 043026437ba63b051df5d14470fe69ce8fa8825c
> > > Author: Hao Ge <hao.ge@linux.dev>
> > > 
> > > mm/vmalloc: undo partial mappings inside the mapping functions
> > > 
> > > This commit adds rollback logic using __vunmap_range_noflush() and
> > > kmsan_vunmap_range_noflush() into vmap_page_range(),
> > > __vmap_pages_range_noflush(), and vmap_pages_range_noflush() when page
> > > table installation or KMSAN metadata setup fails. It also adds range and
> > > page_shift validation in __vmap_pages_range_noflush().
> > > 
> > > > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > > > index 44e0eb32708f1..75aa5acf55660 100644
> > > > --- a/mm/vmalloc.c
> > > > +++ b/mm/vmalloc.c
> > > > @@ -363,6 +363,9 @@ int vmap_page_range(unsigned long addr, unsigned long end,
> > > >   	if (!err)
> > > >   		err = kmsan_ioremap_page_range(addr, end, phys_addr, prot,
> > > >   					       ioremap_max_page_shift);
> > > > +	if (err)
> > > > +		__vunmap_range_noflush(addr, end);
> > > > +
> > > >   	return err;
> > > >   }
> > > [Severity: High]
> > > What happens here when vmap_range_noflush() returns -EINVAL for a range
> > > where end - addr is not PAGE_SIZE-aligned?
> > > 
> > > In vmap_pte_range(), commit a739e6b557af09 added a check to bail out on
> > > unaligned ranges specifically to avoid an infinite loop:
> > > 
> > > mm/vmalloc.c:vmap_pte_range() {
> > > 	...
> > > 	if (WARN_ON_ONCE(!PAGE_ALIGNED(end - addr)))
> > > 		return -EINVAL;
> > > }
> > > 
> > > This can be reached on 64KB page kernels via pci_remap_iospace() ->
> > > vmap_page_range() with 16KB/32KB/48KB PCI I/O ranges (for example via
> > > loongson_pci_probe() or acpi_pci_root_remap_iospace()).
> > > 
> > > When vmap_pte_range() returns -EINVAL, vmap_page_range() now calls
> > > __vunmap_range_noflush(addr, end) with the unaligned range. Whenever a PMD
> > > in [addr, end) is already populated with a PTE table (such as the
> > > PCI_IOBASE PMD populated during early boot by reserve_pio_range(), or an
> > > earlier PMD mapped in the same call), __vunmap_range_noflush() descends
> > > into vunmap_pte_range(), which has no alignment check:
> > > 
> > > mm/vmalloc.c:vunmap_pte_range() {
> > > 	...
> > > 	do {
> > > 		...
> > > 			ptent = ptep_get_and_clear(&init_mm, addr, pte);
> > > 		WARN_ON(!pte_none(ptent) && !pte_present(ptent));
> > > 	} while (pte += (size >> PAGE_SHIFT), addr += size, addr != end);
> > > }
> > > 
> > > Because end - addr is not a multiple of PAGE_SIZE, won't addr step past end
> > > and loop infinitely, clearing unrelated kernel PTEs and corrupting memory
> > > past the PTE table?
> > > 
> > That sounds reasonable. I will add a separate patch in the next version to fix
> > this problem.
> > 
> > > [Severity: Medium]
> > > Also in vmap_page_range(), flush_cache_vmap(addr, end) is called
> > > unconditionally before checking err and before rolling back with
> > > __vunmap_range_noflush(addr, end):
> > > 
> > > mm/vmalloc.c:vmap_page_range() {
> > > 	err = vmap_range_noflush(addr, end, phys_addr, pgprot_nx(prot),
> > > 				 ioremap_max_page_shift);
> > > 	flush_cache_vmap(addr, end);
> > > 	if (!err)
> > > 		err = kmsan_ioremap_page_range(addr, end, phys_addr, prot,
> > > 					       ioremap_max_page_shift);
> > > 	if (err)
> > > 		__vunmap_range_noflush(addr, end);
> > > 
> > > 	return err;
> > > }
> > > 
> > > On UML, flush_cache_vmap is defined as flush_tlb_kernel_range:
> > > 
> > > arch/um/include/asm/cacheflush.h:
> > > #define flush_cache_vmap flush_tlb_kernel_range
> > > 
> > > When vmap_range_noflush() fails partway through or
> > > kmsan_ioremap_page_range() fails, flush_cache_vmap() runs while the
> > > partially installed PTEs are still pte_present(), causing
> > > update_pte_range() in arch/um/kernel/tlb.c to map them into the host OS via
> > > ops->mmap() and clear _PAGE_NEEDSYNC:
> > > 
> > > arch/um/kernel/tlb.c:update_pte_range() {
> > > 	...
> > > 		if (pte_present(*pte)) {
> > > 			...
> > > 			ret = ops->mmap(ops->mm_idp, addr, PAGE_SIZE,
> > > 					prot, fd, offset);
> > > 		} else
> > > 			ret = ops->unmap(ops->mm_idp, addr, PAGE_SIZE);
> > > 
> > > 		*pte = pte_mkuptodate(*pte);
> > > }
> > > 
> > > When __vunmap_range_noflush(addr, end) then clears the software PTEs
> > > without a subsequent TLB flush or unmap notification, won't those host OS
> > > mmap mappings be left behind? Should flush_cache_vmap(addr, end) only be
> > > called after both vmap_range_noflush() and kmsan_ioremap_page_range()
> > > succeed?
> > > 
> > That sounds reasonable. I will add this logic in the next version.
> > 
> > > > @@ -683,27 +686,35 @@ int __vmap_pages_range_noflush(unsigned long addr, unsigned long end,
> > > >   		pgprot_t prot, struct page **pages, unsigned int page_shift)
> > > >   {
> > > >   	unsigned int i, nr = (end - addr) >> PAGE_SHIFT;
> > > > +	unsigned long start = addr;
> > > > +	int err = 0;
> > > > +
> > > > +	if (WARN_ON_ONCE(addr >= end))
> > > > +		return -EINVAL;
> > > >   	if (WARN_ON_ONCE(page_shift < PAGE_SHIFT))
> > > >   		return -EINVAL;
> > > >   	if (!IS_ENABLED(CONFIG_HAVE_ARCH_HUGE_VMALLOC) ||
> > > > -			page_shift == PAGE_SHIFT)
> > > > -		return vmap_small_pages_range_noflush(addr, end, prot, pages);
> > > > -
> > > > -	for (i = 0; i < nr; i += 1U << (page_shift - PAGE_SHIFT)) {
> > > > -		int err;
> > > > -
> > > > -		err = vmap_range_noflush(addr, addr + (1UL << page_shift),
> > > > +			page_shift == PAGE_SHIFT) {
> > > > +		err = vmap_small_pages_range_noflush(addr, end, prot, pages);
> > > > +	} else {
> > > > +		for (i = 0; i < nr; i += 1U << (page_shift - PAGE_SHIFT)) {
> > > > +			err = vmap_range_noflush(addr, addr + (1UL << page_shift),
> > > >   					page_to_phys(pages[i]), prot,
> > > >   					page_shift);
> > > > -		if (err)
> > > > -			return err;
> > > > +			if (err)
> > > > +				break;
> > > > -		addr += 1UL << page_shift;
> > > > +			addr += 1UL << page_shift;
> > > > +		}
> > > >   	}
> > > > -	return 0;
> > > > +	/* Undo the PTEs installed before the failure. */
> > > > +	if (err)
> > > > +		__vunmap_range_noflush(start, end);
> > > > +
> > > > +	return err;
> > > >   }
> > > [Severity: High]
> > > Similarly here, __vmap_pages_range_noflush() checks addr >= end, but does
> > > not check PAGE_ALIGNED(addr) or PAGE_ALIGNED(end).
> > > 
> > > If a caller passes a sub-page or unaligned range (for example, 0 < end -
> > > addr < PAGE_SIZE), nr = (end - addr) >> PAGE_SHIFT evaluates to 0, so the
> > > huge-page path still maps nothing and returns 0, while the small-page path
> > > loops infinitely in vmap_pages_pte_range(). And if the huge-page path fails
> > > on an unaligned range, __vunmap_range_noflush(start, end) will hit the same
> > > infinite loop in vunmap_pte_range().
> > > 
> > > Should both vmap_page_range() and __vmap_pages_range_noflush() validate
> > > PAGE_ALIGNED(addr) && PAGE_ALIGNED(end) upfront before attempting to map or
> > > unmap?
> > > 
> > I think We need to check both PAGE_ALIGNED(addr) and PAGE_ALIGNED(end).
> > As an example: if addr is PAGE-aligned, end is not, and end - addr < PAGE_SIZE,
> > then nr becomes zero. On the huge page path, we simply return 0 with no work performed.
> > 
> > At the same time, vmap_pages_pte_range() needs the same guard as commit
> > a739e6b557af ("mm: vmalloc: WARN_ON if mapping size is not PAGE_SIZE aligned"),
> > to protect against unaligned ranges.
> > 
> > So will do next version.
> > 
> > > >   int vmap_pages_range_noflush(unsigned long addr, unsigned long end,
> > > > @@ -715,7 +726,16 @@ int vmap_pages_range_noflush(unsigned long addr, unsigned long end,
> > > >   	if (ret)
> > > >   		return ret;
> > > > -	return __vmap_pages_range_noflush(addr, end, prot, pages, page_shift);
> > > > +
> > > > +	ret = __vmap_pages_range_noflush(addr, end, prot, pages, page_shift);
> > > > +	/*
> > > > +	 * The page tables undo themselves on failure. Tear down the
> > > > +	 * metadata that was fully set up before the mapping failed.
> > > > +	 */
> > > > +	if (ret)
> > > > +		kmsan_vunmap_range_noflush(addr, end);
> > > > +
> > > > +	return ret;
> > > >   }
> > > [Severity: High]
> > > Can calling __vunmap_range_noflush() and kmsan_vunmap_range_noflush() here
> > > trigger a sleeping-in-atomic-context bug when __vmalloc() or kvmalloc() is
> > > called with GFP_ATOMIC or GFP_NOWAIT?
> > > 
> > > __vmalloc_area_node() supports non-blocking allocations by wrapping
> > > __vmap_pages_range() in memalloc_apply_gfp_scope(gfp_mask):
> > > 
> > > mm/vmalloc.c:__vmalloc_area_node() {
> > > 	...
> > > 	flags = memalloc_apply_gfp_scope(gfp_mask);
> > > 	do {
> > > 		ret = __vmap_pages_range(addr, addr + size, prot, area->pages,
> > > 				page_shift, nested_gfp);
> > > 	} while ((gfp_mask & __GFP_NOFAIL) && (ret < 0));
> > > 	memalloc_restore_gfp_scope(flags);
> > > }
> > > 
> > > When a GFP_ATOMIC or GFP_NOWAIT allocation runs in atomic context (such as
> > > under a spinlock, preempt_disable(), or rcu_read_lock()) and page table
> > > allocation fails in vmap_small_pages_range_noflush() after at least one PMD
> > > in [start, end) is populated (or after kmsan_vmap_pages_range_noflush()
> > > populated the KMSAN shadow/origin ranges), __vmap_pages_range_noflush() and
> > > kmsan_vunmap_range_noflush() synchronously call __vunmap_range_noflush().
> > > 
> > > __vunmap_range_noflush() walks the populated PMDs via vunmap_pmd_range(),
> > > which unconditionally calls cond_resched():
> > > 
> > > mm/vmalloc.c:vunmap_pmd_range() {
> > > 	...
> > > 	do {
> > > 		next = pmd_addr_end(addr, end);
> > > 		if (pmd_none_or_clear_bad(pmd))
> > > 			continue;
> > > 		vunmap_pte_range(pmd, addr, next, mask);
> > > 
> > > 		cond_resched();
> > > 	} while (pmd++, addr = next, addr != end);
> > > }
> > > 
> > > Commit 9c47753167a6 ("mm/vmalloc: defer freeing partly initialized
> > > vm_struct") deferred error-path cleanup in __vmalloc_area_node() to a
> > > workqueue specifically because unmapping on failure can sleep. Doesn't
> > > calling __vunmap_range_noflush() synchronously inside
> > > __vmap_pages_range_noflush() and vmap_pages_range_noflush() reintroduce
> > > cond_resched() in atomic context?
> > > 
> > This issue also appears real. One possible approach is to plumb the caller's gfp_mask into vunmap_pmd_range()
> > to conditionally skip cond_resched().
> > Suren, Ulad, do you have any preferred or cleaner way to handle this?
> 
> 
> Alternatively, we can follow the approach from commit 9c47753167a6
> ("mm/vmalloc: defer freeing partly initialized vm_struct"):
> 
> defer the cleanup to a workqueue when |gfp_mask| has GFP_ATOMIC or
> GFP_NOWAIT，and perform cleanup synchronously
> 
> if we are already in process context. Anyway, I will look into this further
> tomorrow.
> 
I will check the AI concern.

--
Uladzislau Rezki