Subject: Re: [PATCH v11 01/13] mm/vmalloc: fix HUGE_VMAP regression by enabling huge pages in vmalloc_to_page

From: Nicholas Piggin <[email-masked]>
Date: Tue, 02 Feb 2021 20:22:35 +1000

Excerpts from Ding Tianhong's message of January 28, 2021 1:13 pm:
> 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 <[email-masked]>
>> Signed-off-by: Nicholas Piggin <[email-masked]>
>> ---
>>  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.

Hey thanks for finding that, not sure why that didn't trigger any CI.

Anyway newer kernels don't have the ptable-*-hack.h headers, but even so 
it still breaks upstream. arm is using some hand-rolled 2-level folding
of its own (which is fair enough because most 32-bit archs were 2 level
at the time I added pgtable-nopud.h header).

This patch seems to at least make it build.

Thanks,
Nick

---
 arch/arm/include/asm/pgtable-3level.h | 2 --
 arch/arm/include/asm/pgtable.h        | 3 +++
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/arm/include/asm/pgtable-3level.h b/arch/arm/include/asm/pgtable-3level.h
index 2b85d175e999..d4edab51a77c 100644
--- a/arch/arm/include/asm/pgtable-3level.h
+++ b/arch/arm/include/asm/pgtable-3level.h
@@ -186,8 +186,6 @@ static inline pte_t pte_mkspecial(pte_t pte)
 
 #define pmd_write(pmd)		(pmd_isclear((pmd), L_PMD_SECT_RDONLY))
 #define pmd_dirty(pmd)		(pmd_isset((pmd), L_PMD_SECT_DIRTY))
-#define pud_page(pud)		pmd_page(__pmd(pud_val(pud)))
-#define pud_write(pud)		pmd_write(__pmd(pud_val(pud)))
 
 #define pmd_hugewillfault(pmd)	(!pmd_young(pmd) || !pmd_write(pmd))
 #define pmd_thp_or_huge(pmd)	(pmd_huge(pmd) || pmd_trans_huge(pmd))
diff --git a/arch/arm/include/asm/pgtable.h b/arch/arm/include/asm/pgtable.h
index c02f24400369..d63a5bb6bd0c 100644
--- a/arch/arm/include/asm/pgtable.h
+++ b/arch/arm/include/asm/pgtable.h
@@ -166,6 +166,9 @@ extern struct page *empty_zero_page;
 
 extern pgd_t swapper_pg_dir[PTRS_PER_PGD];
 
+#define pud_page(pud)		pmd_page(__pmd(pud_val(pud)))
+#define pud_write(pud)		pmd_write(__pmd(pud_val(pud)))
+
 #define pmd_none(pmd)		(!pmd_val(pmd))
 
 static inline pte_t *pmd_page_vaddr(pmd_t pmd)
-- 
2.23.0