asm-generic/tlb: Provide a comment

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Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
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: Nick Piggin <npiggin@gmail.com>
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:
Peter Zijlstra 2018-09-04 10:43:14 +02:00 committed by Ingo Molnar
parent 5e7a8ca319
commit dea2434c23
1 changed files with 116 additions and 3 deletions

View File

@ -22,6 +22,118 @@
#ifdef CONFIG_MMU
/*
* Generic MMU-gather implementation.
*
* The mmu_gather data structure is used by the mm code to implement the
* correct and efficient ordering of freeing pages and TLB invalidations.
*
* This correct ordering is:
*
* 1) unhook page
* 2) TLB invalidate page
* 3) free page
*
* That is, we must never free a page before we have ensured there are no live
* translations left to it. Otherwise it might be possible to observe (or
* worse, change) the page content after it has been reused.
*
* The mmu_gather API consists of:
*
* - tlb_gather_mmu() / tlb_finish_mmu(); start and finish a mmu_gather
*
* Finish in particular will issue a (final) TLB invalidate and free
* all (remaining) queued pages.
*
* - tlb_start_vma() / tlb_end_vma(); marks the start / end of a VMA
*
* Defaults to flushing at tlb_end_vma() to reset the range; helps when
* there's large holes between the VMAs.
*
* - tlb_remove_page() / __tlb_remove_page()
* - tlb_remove_page_size() / __tlb_remove_page_size()
*
* __tlb_remove_page_size() is the basic primitive that queues a page for
* freeing. __tlb_remove_page() assumes PAGE_SIZE. Both will return a
* boolean indicating if the queue is (now) full and a call to
* tlb_flush_mmu() is required.
*
* tlb_remove_page() and tlb_remove_page_size() imply the call to
* tlb_flush_mmu() when required and has no return value.
*
* - tlb_remove_check_page_size_change()
*
* call before __tlb_remove_page*() to set the current page-size; implies a
* possible tlb_flush_mmu() call.
*
* - tlb_flush_mmu() / tlb_flush_mmu_tlbonly() / tlb_flush_mmu_free()
*
* tlb_flush_mmu_tlbonly() - does the TLB invalidate (and resets
* related state, like the range)
*
* tlb_flush_mmu_free() - frees the queued pages; make absolutely
* sure no additional tlb_remove_page()
* calls happen between _tlbonly() and this.
*
* tlb_flush_mmu() - the above two calls.
*
* - mmu_gather::fullmm
*
* A flag set by tlb_gather_mmu() to indicate we're going to free
* the entire mm; this allows a number of optimizations.
*
* - We can ignore tlb_{start,end}_vma(); because we don't
* care about ranges. Everything will be shot down.
*
* - (RISC) architectures that use ASIDs can cycle to a new ASID
* and delay the invalidation until ASID space runs out.
*
* - mmu_gather::need_flush_all
*
* A flag that can be set by the arch code if it wants to force
* flush the entire TLB irrespective of the range. For instance
* x86-PAE needs this when changing top-level entries.
*
* And requires the architecture to provide and implement tlb_flush().
*
* tlb_flush() may, in addition to the above mentioned mmu_gather fields, make
* use of:
*
* - mmu_gather::start / mmu_gather::end
*
* which provides the range that needs to be flushed to cover the pages to
* be freed.
*
* - mmu_gather::freed_tables
*
* set when we freed page table pages
*
* - tlb_get_unmap_shift() / tlb_get_unmap_size()
*
* returns the smallest TLB entry size unmapped in this range
*
* Additionally there are a few opt-in features:
*
* HAVE_RCU_TABLE_FREE
*
* This provides tlb_remove_table(), to be used instead of tlb_remove_page()
* for page directores (__p*_free_tlb()). This provides separate freeing of
* the page-table pages themselves in a semi-RCU fashion (see comment below).
* Useful if your architecture doesn't use IPIs for remote TLB invalidates
* and therefore doesn't naturally serialize with software page-table walkers.
*
* When used, an architecture is expected to provide __tlb_remove_table()
* which does the actual freeing of these pages.
*
* HAVE_RCU_TABLE_INVALIDATE
*
* This makes HAVE_RCU_TABLE_FREE call tlb_flush_mmu_tlbonly() before freeing
* the page-table pages. Required if you use HAVE_RCU_TABLE_FREE and your
* architecture uses the Linux page-tables natively.
*
*/
#define HAVE_GENERIC_MMU_GATHER
#ifdef CONFIG_HAVE_RCU_TABLE_FREE
/*
* Semi RCU freeing of the page directories.
@ -89,14 +201,17 @@ struct mmu_gather_batch {
*/
#define MAX_GATHER_BATCH_COUNT (10000UL/MAX_GATHER_BATCH)
/* struct mmu_gather is an opaque type used by the mm code for passing around
/*
* struct mmu_gather is an opaque type used by the mm code for passing around
* any data needed by arch specific code for tlb_remove_page.
*/
struct mmu_gather {
struct mm_struct *mm;
#ifdef CONFIG_HAVE_RCU_TABLE_FREE
struct mmu_table_batch *batch;
#endif
unsigned long start;
unsigned long end;
/*
@ -131,8 +246,6 @@ struct mmu_gather {
int page_size;
};
#define HAVE_GENERIC_MMU_GATHER
void arch_tlb_gather_mmu(struct mmu_gather *tlb,
struct mm_struct *mm, unsigned long start, unsigned long end);
void tlb_flush_mmu(struct mmu_gather *tlb);