| Message ID | 1503914913-28893-5-git-send-email-wei.w.wang@intel.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Mon 28-08-17 18:08:32, Wei Wang wrote: > This patch adds support to walk through the free page blocks in the > system and report them via a callback function. Some page blocks may > leave the free list after zone->lock is released, so it is the caller's > responsibility to either detect or prevent the use of such pages. > > One use example of this patch is to accelerate live migration by skipping > the transfer of free pages reported from the guest. A popular method used > by the hypervisor to track which part of memory is written during live > migration is to write-protect all the guest memory. So, those pages that > are reported as free pages but are written after the report function > returns will be captured by the hypervisor, and they will be added to the > next round of memory transfer. OK, looks much better. I still have few nits. > +extern void walk_free_mem_block(void *opaque, > + int min_order, > + bool (*report_page_block)(void *, unsigned long, > + unsigned long)); > + please add names to arguments of the prototype > /* > * Free reserved pages within range [PAGE_ALIGN(start), end & PAGE_MASK) > * into the buddy system. The freed pages will be poisoned with pattern > diff --git a/mm/page_alloc.c b/mm/page_alloc.c > index 6d00f74..81eedc7 100644 > --- a/mm/page_alloc.c > +++ b/mm/page_alloc.c > @@ -4762,6 +4762,71 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask) > show_swap_cache_info(); > } > > +/** > + * walk_free_mem_block - Walk through the free page blocks in the system > + * @opaque: the context passed from the caller > + * @min_order: the minimum order of free lists to check > + * @report_page_block: the callback function to report free page blocks page_block has meaning in the core MM which doesn't strictly match its usage here. Moreover we are reporting pfn ranges rather than struct page range. So report_pfn_range would suit better. [...] > + for_each_populated_zone(zone) { > + for (order = MAX_ORDER - 1; order >= min_order; order--) { > + for (mt = 0; !stop && mt < MIGRATE_TYPES; mt++) { > + spin_lock_irqsave(&zone->lock, flags); > + list = &zone->free_area[order].free_list[mt]; > + list_for_each_entry(page, list, lru) { > + pfn = page_to_pfn(page); > + stop = report_page_block(opaque, pfn, > + 1 << order); > + if (stop) > + break; if (stop) { spin_unlock_irqrestore(&zone->lock, flags); return; } would be both easier and less error prone. E.g. You wouldn't pointlessly iterate over remaining orders just to realize there is nothing to be done for those... > + } > + spin_unlock_irqrestore(&zone->lock, flags); > + } > + } > + } > +} > +EXPORT_SYMBOL_GPL(walk_free_mem_block);
On Mon 28-08-17 15:33:26, Michal Hocko wrote: > On Mon 28-08-17 18:08:32, Wei Wang wrote: > > This patch adds support to walk through the free page blocks in the > > system and report them via a callback function. Some page blocks may > > leave the free list after zone->lock is released, so it is the caller's > > responsibility to either detect or prevent the use of such pages. > > > > One use example of this patch is to accelerate live migration by skipping > > the transfer of free pages reported from the guest. A popular method used > > by the hypervisor to track which part of memory is written during live > > migration is to write-protect all the guest memory. So, those pages that > > are reported as free pages but are written after the report function > > returns will be captured by the hypervisor, and they will be added to the > > next round of memory transfer. > > OK, looks much better. I still have few nits. > > > +extern void walk_free_mem_block(void *opaque, > > + int min_order, > > + bool (*report_page_block)(void *, unsigned long, > > + unsigned long)); > > + > > please add names to arguments of the prototype And one more thing. Your callback returns bool and true usually means a success while you are using it to break out from the loop. This is rather confusing. I would expect iterating until false is returned so the opposite than what you have. You could also change this to int and return 0 on success and < 0 to break out.
On 08/28/2017 09:33 PM, Michal Hocko wrote: > On Mon 28-08-17 18:08:32, Wei Wang wrote: >> This patch adds support to walk through the free page blocks in the >> system and report them via a callback function. Some page blocks may >> leave the free list after zone->lock is released, so it is the caller's >> responsibility to either detect or prevent the use of such pages. >> >> One use example of this patch is to accelerate live migration by skipping >> the transfer of free pages reported from the guest. A popular method used >> by the hypervisor to track which part of memory is written during live >> migration is to write-protect all the guest memory. So, those pages that >> are reported as free pages but are written after the report function >> returns will be captured by the hypervisor, and they will be added to the >> next round of memory transfer. > OK, looks much better. I still have few nits. > >> +extern void walk_free_mem_block(void *opaque, >> + int min_order, >> + bool (*report_page_block)(void *, unsigned long, >> + unsigned long)); >> + > please add names to arguments of the prototype > >> /* >> * Free reserved pages within range [PAGE_ALIGN(start), end & PAGE_MASK) >> * into the buddy system. The freed pages will be poisoned with pattern >> diff --git a/mm/page_alloc.c b/mm/page_alloc.c >> index 6d00f74..81eedc7 100644 >> --- a/mm/page_alloc.c >> +++ b/mm/page_alloc.c >> @@ -4762,6 +4762,71 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask) >> show_swap_cache_info(); >> } >> >> +/** >> + * walk_free_mem_block - Walk through the free page blocks in the system >> + * @opaque: the context passed from the caller >> + * @min_order: the minimum order of free lists to check >> + * @report_page_block: the callback function to report free page blocks > page_block has meaning in the core MM which doesn't strictly match its > usage here. Moreover we are reporting pfn ranges rather than struct page > range. So report_pfn_range would suit better. > > [...] >> + for_each_populated_zone(zone) { >> + for (order = MAX_ORDER - 1; order >= min_order; order--) { >> + for (mt = 0; !stop && mt < MIGRATE_TYPES; mt++) { >> + spin_lock_irqsave(&zone->lock, flags); >> + list = &zone->free_area[order].free_list[mt]; >> + list_for_each_entry(page, list, lru) { >> + pfn = page_to_pfn(page); >> + stop = report_page_block(opaque, pfn, >> + 1 << order); >> + if (stop) >> + break; > if (stop) { > spin_unlock_irqrestore(&zone->lock, flags); > return; > } > > would be both easier and less error prone. E.g. You wouldn't pointlessly > iterate over remaining orders just to realize there is nothing to be > done for those... > Yes, that's better, thanks. I will take other suggestions as well. Best, Wei
diff --git a/include/linux/mm.h b/include/linux/mm.h index 46b9ac5..3c4267d 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1835,6 +1835,11 @@ extern void free_area_init_node(int nid, unsigned long * zones_size, unsigned long zone_start_pfn, unsigned long *zholes_size); extern void free_initmem(void); +extern void walk_free_mem_block(void *opaque, + int min_order, + bool (*report_page_block)(void *, unsigned long, + unsigned long)); + /* * Free reserved pages within range [PAGE_ALIGN(start), end & PAGE_MASK) * into the buddy system. The freed pages will be poisoned with pattern diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 6d00f74..81eedc7 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -4762,6 +4762,71 @@ void show_free_areas(unsigned int filter, nodemask_t *nodemask) show_swap_cache_info(); } +/** + * walk_free_mem_block - Walk through the free page blocks in the system + * @opaque: the context passed from the caller + * @min_order: the minimum order of free lists to check + * @report_page_block: the callback function to report free page blocks + * + * If the callback returns 1, stop iterating the list of free page blocks. + * Otherwise, continue to report. + * + * Please note that there are no locking guarantees for the callback and + * that the reported pfn range might be freed or disappear after the + * callback returns so the caller has to be very careful how it is used. + * + * The callback itself must not sleep or perform any operations which would + * require any memory allocations directly (not even GFP_NOWAIT/GFP_ATOMIC) + * or via any lock dependency. It is generally advisable to implement + * the callback as simple as possible and defer any heavy lifting to a + * different context. + * + * There is no guarantee that each free range will be reported only once + * during one walk_free_mem_block invocation. + * + * pfn_to_page on the given range is strongly discouraged and if there is + * an absolute need for that make sure to contact MM people to discuss + * potential problems. + * + * The function itself might sleep so it cannot be called from atomic + * contexts. + * + * In general low orders tend to be very volatile and so it makes more + * sense to query larger ones first for various optimizations which like + * ballooning etc... This will reduce the overhead as well. + */ +void walk_free_mem_block(void *opaque, + int min_order, + bool (*report_page_block)(void *, unsigned long, + unsigned long)) +{ + struct zone *zone; + struct page *page; + struct list_head *list; + int order; + enum migratetype mt; + unsigned long pfn, flags; + bool stop = 0; + + for_each_populated_zone(zone) { + for (order = MAX_ORDER - 1; order >= min_order; order--) { + for (mt = 0; !stop && mt < MIGRATE_TYPES; mt++) { + spin_lock_irqsave(&zone->lock, flags); + list = &zone->free_area[order].free_list[mt]; + list_for_each_entry(page, list, lru) { + pfn = page_to_pfn(page); + stop = report_page_block(opaque, pfn, + 1 << order); + if (stop) + break; + } + spin_unlock_irqrestore(&zone->lock, flags); + } + } + } +} +EXPORT_SYMBOL_GPL(walk_free_mem_block); + static void zoneref_set_zone(struct zone *zone, struct zoneref *zoneref) { zoneref->zone = zone;