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    SubjectRe: [RFC 3/4] lockless vma caching
    From
    Minchan Kim wrote:
    >
    >
    > KAMEZAWA Hiroyuki wrote:
    >> For accessing vma in lockless style, some modification for vma lookup is
    >> required. Now, rb-tree is used and it doesn't allow read while
    >> modification.
    >>
    >> This is a trial to caching vma rather than diving into rb-tree. The last
    >> fault vma is cached to pgd's page->cached_vma field. And, add reference
    >> count
    >> and waitqueue to vma.
    >>
    >> The accessor will have to do
    >>
    >> vma = lookup_vma_cache(mm, address);
    >> if (vma) {
    >> if (mm_check_version(mm) && /* no write lock at this point ? */
    >> (vma->vm_start <= address) && (vma->vm_end > address))
    >> goto found_vma; /* start speculative job */
    >> else
    >> vma_release_cache(vma);
    >> vma = NULL;
    >> }
    >> vma = find_vma();
    >> found_vma:
    >> ....do some jobs....
    >> vma_release_cache(vma);
    >>
    >> Maybe some more consideration for invalidation point is necessary.
    >>
    >> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
    >> ---
    >> include/linux/mm.h | 20 +++++++++
    >> include/linux/mm_types.h | 5 ++
    >> mm/memory.c | 14 ++++++
    >> mm/mmap.c | 102
    >> +++++++++++++++++++++++++++++++++++++++++++++--
    >> mm/page_alloc.c | 1
    >> 5 files changed, 138 insertions(+), 4 deletions(-)
    >>
    >> Index: mmotm-mm-accessor/include/linux/mm.h
    >> ===================================================================
    >> --- mmotm-mm-accessor.orig/include/linux/mm.h
    >> +++ mmotm-mm-accessor/include/linux/mm.h
    >> @@ -763,6 +763,26 @@ unsigned long unmap_vmas(struct mmu_gath
    >> unsigned long end_addr, unsigned long *nr_accounted,
    >> struct zap_details *);
    >>
    >> +struct vm_area_struct *lookup_vma_cache(struct mm_struct *mm,
    >> + unsigned long address);
    >> +void invalidate_vma_cache(struct mm_struct *mm,
    >> + struct vm_area_struct *vma);
    >> +void wait_vmas_cache_range(struct vm_area_struct *vma, unsigned long
    >> end);
    >> +
    >> +static inline void vma_hold(struct vm_area_struct *vma)
    > Nitpick:
    > How about static inline void vma_cache_[get/put] naming?
    >
    Hmm. yes, put/get is popular name for this kind of counters.
    Why I don't use put/get name is that this counter's purpose is
    for helping cache. So, I avoided popular name.
    I may change my mind in the next version ;)

    >> +{
    >> + atomic_inc(&vma->cache_access);
    >> +}
    >> +
    >> +void __vma_release(struct vm_area_struct *vma);
    >> +static inline void vma_release(struct vm_area_struct *vma)
    >> +{
    >> + if (atomic_dec_and_test(&vma->cache_access)) {
    >> + if (waitqueue_active(&vma->cache_wait))
    >> + __vma_release(vma);
    >> + }
    >> +}
    >> +
    >> /**
    >> * mm_walk - callbacks for walk_page_range
    >> * @pgd_entry: if set, called for each non-empty PGD (top-level) entry
    >> Index: mmotm-mm-accessor/include/linux/mm_types.h
    >> ===================================================================
    >> --- mmotm-mm-accessor.orig/include/linux/mm_types.h
    >> +++ mmotm-mm-accessor/include/linux/mm_types.h
    >> @@ -12,6 +12,7 @@
    >> #include <linux/completion.h>
    >> #include <linux/cpumask.h>
    >> #include <linux/page-debug-flags.h>
    >> +#include <linux/wait.h>
    >> #include <asm/page.h>
    >> #include <asm/mmu.h>
    >>
    >> @@ -77,6 +78,7 @@ struct page {
    >> union {
    >> pgoff_t index; /* Our offset within mapping. */
    >> void *freelist; /* SLUB: freelist req. slab lock */
    >> + void *cache;
    >
    > Let's add annotation "/* vm_area_struct cache when the page is used as
    > page table */".
    >
    ok.

    >
    >> };
    >> struct list_head lru; /* Pageout list, eg. active_list
    >> * protected by zone->lru_lock !
    >> @@ -180,6 +182,9 @@ struct vm_area_struct {
    >> void * vm_private_data; /* was vm_pte (shared mem) */
    >> unsigned long vm_truncate_count;/* truncate_count or restart_addr */
    >>
    >> + atomic_t cache_access;
    >> + wait_queue_head_t cache_wait;
    >> +
    >> #ifndef CONFIG_MMU
    >> struct vm_region *vm_region; /* NOMMU mapping region */
    >> #endif
    >> Index: mmotm-mm-accessor/mm/memory.c
    >> ===================================================================
    >> --- mmotm-mm-accessor.orig/mm/memory.c
    >> +++ mmotm-mm-accessor/mm/memory.c
    >> @@ -145,6 +145,14 @@ void pmd_clear_bad(pmd_t *pmd)
    >> pmd_clear(pmd);
    >> }
    >>
    >
    > Let's put the note here. "The caller needs to hold the pte lock"
    >
    Sure.

    >> +static void update_vma_cache(pmd_t *pmd, struct vm_area_struct *vma)
    >> +{
    >> + struct page *page;
    >> + /* ptelock is held */
    >> + page = pmd_page(*pmd);
    >> + page->cache = vma;
    >> +}
    >> +
    >> /*
    >> * Note: this doesn't free the actual pages themselves. That
    >> * has been handled earlier when unmapping all the memory regions.
    >> @@ -2118,6 +2126,7 @@ reuse:
    >> if (ptep_set_access_flags(vma, address, page_table, entry,1))
    >> update_mmu_cache(vma, address, entry);
    >> ret |= VM_FAULT_WRITE;
    >> + update_vma_cache(pmd, vma);
    >> goto unlock;
    >> }
    >>
    > ..
    > <snip>
    > ..
    >
    >> Index: mmotm-mm-accessor/mm/page_alloc.c
    >> ===================================================================
    >> --- mmotm-mm-accessor.orig/mm/page_alloc.c
    >> +++ mmotm-mm-accessor/mm/page_alloc.c
    >> @@ -698,6 +698,7 @@ static int prep_new_page(struct page *pa
    >>
    >> set_page_private(page, 0);
    >> set_page_refcounted(page);
    >> + page->cache = NULL;
    >
    > Is here is proper place to initialize page->cache?
    > It cause unnecessary overhead about not pmd page.
    >
    > How about pmd_alloc?
    >
    The macro pmd_xxx was complicated and scattered over headers,
    so I clear it here.
    But yes, you're right. I'll do so when I write a patch, not-for-trial.

    Thanks,
    -Kame



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