Minki/linux
1
0
Fork 1
mirror of https://github.com/torvalds/linux.git synced 2024-12-01 00:21:32 +00:00
Code Issues Packages Projects Releases Wiki Activity

x86/mm: Clean up the pmd_read_atomic() comments

Browse Source 
Fix spelling, consistent parenthesis and grammar - and also clarify
the language where needed.

Reviewed-by: Wei Yang <richardw.yang@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Ingo Molnar 2019-09-25 08:38:57 +02:00
parent a2f7a0bfca
commit 44e09568cf
1 changed files with 23 additions and 21 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

44
arch/x86/include/asm/pgtable-3level.h
View File

@ -36,39 +36,41 @@ static inline void native_set_pte(pte_t *ptep, pte_t pte)
#define pmd_read_atomic pmd_read_atomic
/*
* pte_offset_map_lock on 32bit PAE kernels was reading the pmd_t with
* a "*pmdp" dereference done by gcc. Problem is, in certain places
* where pte_offset_map_lock is called, concurrent page faults are
* pte_offset_map_lock() on 32-bit PAE kernels was reading the pmd_t with
* a "*pmdp" dereference done by GCC. Problem is, in certain places
* where pte_offset_map_lock() is called, concurrent page faults are
* allowed, if the mmap_sem is hold for reading. An example is mincore
* vs page faults vs MADV_DONTNEED. On the page fault side
* pmd_populate rightfully does a set_64bit, but if we're reading the
* pmd_populate() rightfully does a set_64bit(), but if we're reading the
* pmd_t with a "*pmdp" on the mincore side, a SMP race can happen
* because gcc will not read the 64bit of the pmd atomically. To fix
* this all places running pte_offset_map_lock() while holding the
* because GCC will not read the 64-bit value of the pmd atomically.
*
* To fix this all places running pte_offset_map_lock() while holding the
* mmap_sem in read mode, shall read the pmdp pointer using this
* function to know if the pmd is null nor not, and in turn to know if
* function to know if the pmd is null or not, and in turn to know if
* they can run pte_offset_map_lock() or pmd_trans_huge() or other pmd
* operations.
*
* Without THP if the mmap_sem is hold for reading, the pmd can only
* transition from null to not null while pmd_read_atomic runs. So
* Without THP if the mmap_sem is held for reading, the pmd can only
* transition from null to not null while pmd_read_atomic() runs. So
* we can always return atomic pmd values with this function.
*
* With THP if the mmap_sem is hold for reading, the pmd can become
* With THP if the mmap_sem is held for reading, the pmd can become
* trans_huge or none or point to a pte (and in turn become "stable")
* at any time under pmd_read_atomic. We could read it really
* atomically here with a atomic64_read for the THP enabled case (and
* at any time under pmd_read_atomic(). We could read it truly
* atomically here with an atomic64_read() for the THP enabled case (and
* it would be a whole lot simpler), but to avoid using cmpxchg8b we
* only return an atomic pmdval if the low part of the pmdval is later
* found stable (i.e. pointing to a pte). And we're returning a none
* pmdval if the low part of the pmd is none. In some cases the high
* and low part of the pmdval returned may not be consistent if THP is
* enabled (the low part may point to previously mapped hugepage,
* while the high part may point to a more recently mapped hugepage),
* but pmd_none_or_trans_huge_or_clear_bad() only needs the low part
* of the pmd to be read atomically to decide if the pmd is unstable
* or not, with the only exception of when the low part of the pmd is
* zero in which case we return a none pmd.
* found to be stable (i.e. pointing to a pte). We are also returning a
* 'none' (zero) pmdval if the low part of the pmd is zero.
*
* In some cases the high and low part of the pmdval returned may not be
* consistent if THP is enabled (the low part may point to previously
* mapped hugepage, while the high part may point to a more recently
* mapped hugepage), but pmd_none_or_trans_huge_or_clear_bad() only
* needs the low part of the pmd to be read atomically to decide if the
* pmd is unstable or not, with the only exception when the low part
* of the pmd is zero, in which case we return a 'none' pmd.
*/
static inline pmd_t pmd_read_atomic(pmd_t *pmdp)
{