Messages in this thread | | | Date | Fri, 6 Jun 2008 18:05:14 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH -mm 16/25] SHM_LOCKED pages are non-reclaimable |
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On Fri, 06 Jun 2008 16:28:54 -0400 Rik van Riel <riel@redhat.com> wrote:
> From: Lee Schermerhorn <Lee.Schermerhorn@hp.com> > > Against: 2.6.26-rc2-mm1 > > While working with Nick Piggin's mlock patches,
Change log refers to information which its reader has not got a hope of actually locating.
> I noticed that > shmem segments locked via shmctl(SHM_LOCKED) were not being handled. > SHM_LOCKed pages work like ramdisk pages
Well, OK. As long as one remembers that "ramdisk pages" are different from "pages of a file which is on ramdisk". Tricky, huh?
> --the writeback function > just redirties the page so that it can't be reclaimed. Deal with > these using the same approach as for ram disk pages. > > Use the AS_NORECLAIM flag to mark address_space of SHM_LOCKed > shared memory regions as non-reclaimable. Then these pages > will be culled off the normal LRU lists during vmscan.
So I guess there's more justification for handling these pages in this manner, because someone could come along later and unlock them. But that isn't true of /dev/ram0 pages and ramfs pages, etc.
> Add new wrapper function to clear the mapping's noreclaim state > when/if shared memory segment is munlocked. > > Add 'scan_mapping_noreclaim_page()' to mm/vmscan.c to scan all > pages in the shmem segment's mapping [struct address_space] for > reclaimability now that they're no longer locked. If so, move > them to the appropriate zone lru list. Note that > scan_mapping_noreclaim_page() must be able to sleep on page_lock(), > so we can't call it holding the shmem info spinlock nor the shmid > spinlock. So, we pass the mapping [address_space] back to shmctl() > on SHM_UNLOCK for rescuing any nonreclaimable pages after dropping > the spinlocks. Once we drop the shmid lock, the backing shmem file > can be deleted if the calling task doesn't have the shm area > attached. To handle this, we take an extra reference on the file > before dropping the shmid lock and drop the reference after scanning > the mapping's noreclaim pages. > > > ... > > + > +/** > + * check_move_noreclaim_page - check page for reclaimability and move to appropriate zone lru list > + * @page: page to check reclaimability and move to appropriate lru list > + * @zone: zone page is in > + * > + * Checks a page for reclaimability and moves the page to the appropriate > + * zone lru list. > + * > + * Restrictions: zone->lru_lock must be held, page must be on LRU and must > + * have PageNoreclaim set. > + */ > +static void check_move_noreclaim_page(struct page *page, struct zone *zone) > +{ > + > + ClearPageNoreclaim(page); /* for page_reclaimable() */
Confused. Didn't we just lose track of our NR_NORECLAIM accounting?
> + if (page_reclaimable(page, NULL)) { > + enum lru_list l = LRU_INACTIVE_ANON + page_file_cache(page); > + __dec_zone_state(zone, NR_NORECLAIM); > + list_move(&page->lru, &zone->list[l]); > + __inc_zone_state(zone, NR_INACTIVE_ANON + l); > + } else { > + /* > + * rotate noreclaim list > + */ > + SetPageNoreclaim(page); > + list_move(&page->lru, &zone->list[LRU_NORECLAIM]); > + } > +} > + > +/** > + * scan_mapping_noreclaim_pages - scan an address space for reclaimable pages > + * @mapping: struct address_space to scan for reclaimable pages > + * > + * Scan all pages in mapping. Check non-reclaimable pages for > + * reclaimability and move them to the appropriate zone lru list. > + */ > +void scan_mapping_noreclaim_pages(struct address_space *mapping) > +{ > + pgoff_t next = 0; > + pgoff_t end = (i_size_read(mapping->host) + PAGE_CACHE_SIZE - 1) >> > + PAGE_CACHE_SHIFT; > + struct zone *zone; > + struct pagevec pvec; > + > + if (mapping->nrpages == 0) > + return; > + > + pagevec_init(&pvec, 0); > + while (next < end && > + pagevec_lookup(&pvec, mapping, next, PAGEVEC_SIZE)) { > + int i; > + > + zone = NULL; > + > + for (i = 0; i < pagevec_count(&pvec); i++) { > + struct page *page = pvec.pages[i]; > + pgoff_t page_index = page->index; > + struct zone *pagezone = page_zone(page); > + > + if (page_index > next) > + next = page_index; > + next++; > + > + if (TestSetPageLocked(page)) { > + /* > + * OK, let's do it the hard way... > + */ > + if (zone) > + spin_unlock_irq(&zone->lru_lock); > + zone = NULL; > + lock_page(page); > + } > + > + if (pagezone != zone) { > + if (zone) > + spin_unlock_irq(&zone->lru_lock); > + zone = pagezone; > + spin_lock_irq(&zone->lru_lock); > + } > + > + if (PageLRU(page) && PageNoreclaim(page)) > + check_move_noreclaim_page(page, zone); > + > + unlock_page(page); > + > + } > + if (zone) > + spin_unlock_irq(&zone->lru_lock); > + pagevec_release(&pvec); > + } > + > +}
This function can spend fantastically large amounts of time under spin_lock_irq().
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