Re: [PATCH 3/3] mm/vmalloc: fix vmalloc_to_page for huge vmap mappings
From: Anshuman Khandual <hidden>
Date: 2019-06-24 06:52:42
Also in:
linux-mm, linuxppc-dev
On 06/23/2019 03:14 PM, Nicholas Piggin wrote:
quoted hunk ↗ jump to hunk
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") Signed-off-by: Nicholas Piggin <npiggin@gmail.com> --- include/asm-generic/4level-fixup.h | 1 + include/asm-generic/5level-fixup.h | 1 + mm/vmalloc.c | 37 +++++++++++++++++++----------- 3 files changed, 26 insertions(+), 13 deletions(-)diff --git a/include/asm-generic/4level-fixup.h b/include/asm-generic/4level-fixup.h index e3667c9a33a5..3cc65a4dd093 100644 --- a/include/asm-generic/4level-fixup.h +++ b/include/asm-generic/4level-fixup.h@@ -20,6 +20,7 @@ #define pud_none(pud) 0 #define pud_bad(pud) 0 #define pud_present(pud) 1 +#define pud_large(pud) 0 #define pud_ERROR(pud) do { } while (0) #define pud_clear(pud) pgd_clear(pud) #define pud_val(pud) pgd_val(pud)diff --git a/include/asm-generic/5level-fixup.h b/include/asm-generic/5level-fixup.h index bb6cb347018c..c4377db09a4f 100644 --- a/include/asm-generic/5level-fixup.h +++ b/include/asm-generic/5level-fixup.h@@ -22,6 +22,7 @@ #define p4d_none(p4d) 0 #define p4d_bad(p4d) 0 #define p4d_present(p4d) 1 +#define p4d_large(p4d) 0 #define p4d_ERROR(p4d) do { } while (0) #define p4d_clear(p4d) pgd_clear(p4d) #define p4d_val(p4d) pgd_val(p4d)diff --git a/mm/vmalloc.c b/mm/vmalloc.c index 4c9e150e5ad3..4be98f700862 100644 --- a/mm/vmalloc.c +++ b/mm/vmalloc.c@@ -36,6 +36,7 @@ #include <linux/rbtree_augmented.h> #include <linux/uaccess.h> +#include <asm/pgtable.h> #include <asm/tlbflush.h> #include <asm/shmparam.h>@@ -284,26 +285,36 @@ struct page *vmalloc_to_page(const void *vmalloc_addr) if (pgd_none(*pgd)) return NULL; + p4d = p4d_offset(pgd, addr); if (p4d_none(*p4d)) return NULL; - pud = pud_offset(p4d, addr); + if (WARN_ON_ONCE(p4d_bad(*p4d))) + return NULL;
The warning here is a required addition but it needs to be moved after p4d_large() check. Please see the next comment below.
+#ifdef CONFIG_HAVE_ARCH_HUGE_VMAP + if (p4d_large(*p4d)) + return p4d_page(*p4d) + ((addr & ~P4D_MASK) >> PAGE_SHIFT); +#endif - /* - * 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 (WARN_ON_ONCE(pud_bad(*pud))) return NULL; +#ifdef CONFIG_HAVE_ARCH_HUGE_VMAP + if (pud_large(*pud)) + return pud_page(*pud) + ((addr & ~PUD_MASK) >> PAGE_SHIFT); +#endif +
pud_bad() on arm64 returns true when the PUD does not point to a next page table page implying the fact that it might be a large/huge entry. I am not sure if the semantics holds good for other architectures too. But on arm64 if pud_large() is true, then pud_bad() will be true as well. So pud_bad() check must happen after pud_large() check. So the sequence here should be 1. pud_none() --> Nothing is in here, return NULL 2. pud_large() --> Return offset page address from the huge page mapping 3. pud_bad() --> Return NULL as there is no more page table level left Checking pud_bad() first can return NULL for a valid huge mapping.
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 (WARN_ON_ONCE(pmd_bad(*pmd))) return NULL; +#ifdef CONFIG_HAVE_ARCH_HUGE_VMAP + if (pmd_large(*pmd)) + return pmd_page(*pmd) + ((addr & ~PMD_MASK) >> PAGE_SHIFT); +#endif
Ditto. I see that your previous proposal had this right which got changed in this manner after my comments. Sorry about it. It was recently when I learned (correctly) that expected semantics of pxx_bad() is that - It does not point to the next page table page. Hence I wonder why is this not renamed as pxx_table() instead to make it absolutely clear. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel