Re: Regression from a9b3c355c2e6 ("asm-generic: pgalloc: provide generic __pgd_{alloc,free}") with CONFIG_DEBUG_VM_PGFLAGS=y and Xen

[Matthew Wilcox <willy@infradead.org>]

On Wed, Apr 16, 2025 at 02:07:20PM +0200, Petr Vaněk wrote:
> I have discovered a regression introduced in commit a9b3c355c2e6
> ("asm-generic: pgalloc: provide generic __pgd_{alloc,free}") [1,2] in
> kernel version 6.14. The problem occurs when the x86 kernel is
> configured with CONFIG_DEBUG_VM_PGFLAGS=y and is run as a PV Dom0 in Xen
> 4.19.1. During the startup, the kernel panics with the error log below.

You also have to have CONFIG_MITIGATION_PAGE_TABLE_ISOLATION enabled
to hit this problem, otherwise we allocate an order-0 page.

> The commit changed PGD allocation path.  In the new implementation
> _pgd_alloc allocates memory with __pgd_alloc, which indirectly calls 
> 
>   alloc_pages_noprof(gfp | __GFP_COMP, order);
> 
> This is in contrast to the old behavior, where __get_free_pages was
> used, which indirectly called
> 
>   alloc_pages_noprof(gfp_mask & ~__GFP_HIGHMEM, order);
> 
> The key difference is that the new allocator can return a compound page.
> When xen_pin_page is later called on such a page, it call
> TestSetPagePinned function, which internally uses the PF_NO_COMPOUND
> macro. This macro enforces VM_BUG_ON_PGFLAGS if PageCompound is true,
> triggering the panic when CONFIG_DEBUG_VM_PGFLAGS is enabled.

I suspect the right thing to do here is to change the PF_NO_COMPOUND to
PF_HEAD.  Probably for all of these:

/* Xen */
PAGEFLAG(Pinned, pinned, PF_NO_COMPOUND)
        TESTSCFLAG(Pinned, pinned, PF_NO_COMPOUND)
PAGEFLAG(SavePinned, savepinned, PF_NO_COMPOUND);
PAGEFLAG(Foreign, foreign, PF_NO_COMPOUND);
PAGEFLAG(XenRemapped, xen_remapped, PF_NO_COMPOUND)
        TESTCLEARFLAG(XenRemapped, xen_remapped, PF_NO_COMPOUND)

Could you give that a try?