Re: [PATCH v7 4/7] Drivers: hv: Fix huge page handling in memory region traversal

[Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>]

On Thu, Dec 11, 2025 at 05:37:26PM +0000, Michael Kelley wrote:
> From: Stanislav Kinsburskii <skinsburskii@linux.microsoft.com> Sent: Thursday, December 4, 2025 1:09 PM

<snip>


> I've been playing around with mmu notifiers and 2 Meg pages. At least in my
> experiment, there's a case where the .invalidate callback is invoked on a
> range *before* the 2 Meg page is split. The kernel code that does this is
> in zap_page_range_single_batched(). Early on this function calls
> mmu_notifier_invalidate_range_start(), which invokes the .invalidate
> callback on the initial range. Later on, unmap_single_vma() is called, which
> does the split and eventually makes a second .invalidate callback for the
> entire 2 Meg page.
> 
> Details:  My experiment is a user space program that does the following:
> 
> 1. Allocates 16 Megs of memory on a 16 Meg boundary using
> posix_memalign(). So this is private anonymous memory. Transparent
> huge pages are enabled.
> 
> 2. Writes to a byte in each 4K page so they are all populated. 
> /proc/meminfo shows eight 2 Meg pages have been allocated.
> 
> 3. Creates an mmu notifier for the allocated 16 Megs, using an ioctl
> hacked into the kernel for experimentation purposes.
> 
> 4. Uses madvise() with the DONTNEED option to free 32 Kbytes on a 4K
> page boundary somewhere in the 16 Meg allocation. This results in an mmu
> notifier invalidate callback for that 32 Kbytes. Then there's a second invalidate
> callback covering the entire 2 Meg page that contains the 32 Kbyte range.
> Kernel stack traces for the two invalidate callbacks show them originating
> in zap_page_range_single_batched().
> 
> 5. Sleeps for 60 seconds. During that time, khugepaged wakes up and does
> hpage_collapse_scan_pmd() -> collapse_huge_page(), which generates a third
> .invalidate callback for the 2 Meg page. I'm haven't investigated what this is
> all about.
> 
> 6. Interestingly, if Step 4 above does a slightly different operation using
> mprotect() with PROT_READ instead of madvise(), the 2 Meg page is split first.
> The .invalidate callback for the full 2 Meg happens before the .invalidate
> callback for the specified range.
> 
> The root partition probably isn't doing madvise() with DONTNEED for memory
> allocated for guests. But regardless of what user space does or doesn't do, MSHV's
> invalidate callback path should be made safe for this case. Maybe that's just
> detecting it and returning an error (and maybe a WARN_ON) if user space
> doesn't need it to work.
> 
> Michael
> 

The issue is addressed by "mshv: Align huge page stride with guest
mapping" patch.

Thanks a lot once again for your help in identifying it,
Stanislav