linux-kernel - Re: [PATCH v11 01/13] mm/vmalloc: fix HUGE_VMAP regression by enabling huge pages in vmalloc_to

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Date:   Thu, 28 Jan 2021 11:13:20 +0800
From:   Ding Tianhong <dingtianhong@...wei.com>
To:     Nicholas Piggin <npiggin@...il.com>, <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     <linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>,
        <linuxppc-dev@...ts.ozlabs.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v11 01/13] mm/vmalloc: fix HUGE_VMAP regression by
 enabling huge pages in vmalloc_to_page

On 2021/1/26 12:44, Nicholas Piggin wrote:
> vmalloc_to_page returns NULL for addresses mapped by larger pages[*].
> Whether or not a vmap is huge depends on the architecture details,
> alignments, boot options, etc., which the caller can not be expected
> to know. Therefore HUGE_VMAP is a regression for vmalloc_to_page.
> 
> This change teaches vmalloc_to_page about larger pages, and returns
> the struct page that corresponds to the offset within the large page.
> This makes the API agnostic to mapping implementation details.
> 
> [*] As explained by commit 029c54b095995 ("mm/vmalloc.c: huge-vmap:
>     fail gracefully on unexpected huge vmap mappings")
> 
> Reviewed-by: Christoph Hellwig <hch@....de>
> Signed-off-by: Nicholas Piggin <npiggin@...il.com>
> ---
>  mm/vmalloc.c | 41 ++++++++++++++++++++++++++---------------
>  1 file changed, 26 insertions(+), 15 deletions(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index e6f352bf0498..62372f9e0167 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -34,7 +34,7 @@
>  #include <linux/bitops.h>
>  #include <linux/rbtree_augmented.h>
>  #include <linux/overflow.h>
> -
> +#include <linux/pgtable.h>
>  #include <linux/uaccess.h>
>  #include <asm/tlbflush.h>
>  #include <asm/shmparam.h>
> @@ -343,7 +343,9 @@ int is_vmalloc_or_module_addr(const void *x)
>  }
>  
>  /*
> - * Walk a vmap address to the struct page it maps.
> + * Walk a vmap address to the struct page it maps. Huge vmap mappings will
> + * return the tail page that corresponds to the base page address, which
> + * matches small vmap mappings.
>   */
>  struct page *vmalloc_to_page(const void *vmalloc_addr)
>  {
> @@ -363,25 +365,33 @@ struct page *vmalloc_to_page(const void *vmalloc_addr)
>  
>  	if (pgd_none(*pgd))
>  		return NULL;
> +	if (WARN_ON_ONCE(pgd_leaf(*pgd)))
> +		return NULL; /* XXX: no allowance for huge pgd */
> +	if (WARN_ON_ONCE(pgd_bad(*pgd)))
> +		return NULL;
> +
>  	p4d = p4d_offset(pgd, addr);
>  	if (p4d_none(*p4d))
>  		return NULL;
> -	pud = pud_offset(p4d, addr);
> +	if (p4d_leaf(*p4d))
> +		return p4d_page(*p4d) + ((addr & ~P4D_MASK) >> PAGE_SHIFT);
> +	if (WARN_ON_ONCE(p4d_bad(*p4d)))
> +		return NULL;
>  
> -	/*
> -	 * Don't dereference bad PUD or PMD (below) entries. This will also
> -	 * identify huge mappings, which we may encounter on architectures
> -	 * that define CONFIG_HAVE_ARCH_HUGE_VMAP=y. Such regions will be
> -	 * identified as vmalloc addresses by is_vmalloc_addr(), but are
> -	 * not [unambiguously] associated with a struct page, so there is
> -	 * no correct value to return for them.
> -	 */
> -	WARN_ON_ONCE(pud_bad(*pud));
> -	if (pud_none(*pud) || pud_bad(*pud))
> +	pud = pud_offset(p4d, addr);
> +	if (pud_none(*pud))
> +		return NULL;
> +	if (pud_leaf(*pud))
> +		return pud_page(*pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT);

Hi Nicho:

/builds/1mzfdQzleCy69KZFb5qHNSEgabZ/mm/vmalloc.c: In function 'vmalloc_to_page':
/builds/1mzfdQzleCy69KZFb5qHNSEgabZ/include/asm-generic/pgtable-nop4d-hack.h:48:27: error: implicit declaration of function 'pud_page'; did you mean 'put_page'? [-Werror=implicit-function-declaration]
   48 | #define pgd_page(pgd)    (pud_page((pud_t){ pgd }))
      |                           ^~~~~~~~

the pug_page is not defined for aarch32 when enabling 2-level page config, it break the system building.


> +	if (WARN_ON_ONCE(pud_bad(*pud)))
>  		return NULL;
> +
>  	pmd = pmd_offset(pud, addr);
> -	WARN_ON_ONCE(pmd_bad(*pmd));
> -	if (pmd_none(*pmd) || pmd_bad(*pmd))
> +	if (pmd_none(*pmd))
> +		return NULL;
> +	if (pmd_leaf(*pmd))
> +		return pmd_page(*pmd) + ((addr & ~PMD_MASK) >> PAGE_SHIFT);
> +	if (WARN_ON_ONCE(pmd_bad(*pmd)))
>  		return NULL;
>  
>  	ptep = pte_offset_map(pmd, addr);
> @@ -389,6 +399,7 @@ struct page *vmalloc_to_page(const void *vmalloc_addr)
>  	if (pte_present(pte))
>  		page = pte_page(pte);
>  	pte_unmap(ptep);
> +
>  	return page;
>  }
>  EXPORT_SYMBOL(vmalloc_to_page);
>