| Message ID | 1588130803-20527-1-git-send-email-iamjoonsoo.kim@lge.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | change the implementation of the PageHighMem() | expand |
On Wed, 29 Apr 2020 12:26:33 +0900 js1304@gmail.com wrote: > From: Joonsoo Kim <iamjoonsoo.kim@lge.com> > > Changes on v2 > - add "acked-by", "reviewed-by" tags > - replace PageHighMem() with use open-code, instead of using > new PageHighMemZone() macro. Related file is "include/linux/migrate.h" > > Hello, > > This patchset separates two use cases of PageHighMem() by introducing > PageHighMemZone() macro. And, it changes the implementation of > PageHighMem() to reflect the actual meaning of this macro. This patchset > is a preparation step for the patchset, > "mm/cma: manage the memory of the CMA area by using the ZONE_MOVABLE" [1]. > > PageHighMem() is used for two different cases. One is to check if there > is a direct mapping for this page or not. The other is to check the > zone of this page, that is, weather it is the highmem type zone or not. > > Until now, both the cases are the perfectly same thing. So, implementation > of the PageHighMem() uses the one case that checks if the zone of the page > is the highmem type zone or not. > > "#define PageHighMem(__p) is_highmem_idx(page_zonenum(__p))" > > ZONE_MOVABLE is special. It is considered as normal type zone on > !CONFIG_HIGHMEM, but, it is considered as highmem type zone > on CONFIG_HIGHMEM. Let's focus on later case. In later case, all pages > on the ZONE_MOVABLE has no direct mapping until now. > > However, following patchset > "mm/cma: manage the memory of the CMA area by using the ZONE_MOVABLE" > , which is once merged and reverted, will be tried again and will break > this assumption that all pages on the ZONE_MOVABLE has no direct mapping. > Hence, the ZONE_MOVABLE which is considered as highmem type zone could > have the both types of pages, direct mapped and not. Since > the ZONE_MOVABLE could have both type of pages, __GFP_HIGHMEM is still > required to allocate the memory from it. And, we conservatively need to > consider the ZONE_MOVABLE as highmem type zone. > > Even in this situation, PageHighMem() for the pages on the ZONE_MOVABLE > when it is called for checking the direct mapping should return correct > result. Current implementation of PageHighMem() just returns TRUE > if the zone of the page is on a highmem type zone. So, it could be wrong > if the page on the MOVABLE_ZONE is actually direct mapped. > > To solve this potential problem, this patch introduces a new > PageHighMemZone() macro. In following patches, two use cases of > PageHighMem() are separated by calling proper macro, PageHighMem() and > PageHighMemZone(). Then, implementation of PageHighMem() will be changed > as just checking if the direct mapping exists or not, regardless of > the zone of the page. > > Note that there are some rules to determine the proper macro. > > 1. If PageHighMem() is called for checking if the direct mapping exists > or not, use PageHighMem(). > 2. If PageHighMem() is used to predict the previous gfp_flags for > this page, use PageHighMemZone(). The zone of the page is related to > the gfp_flags. > 3. If purpose of calling PageHighMem() is to count highmem page and > to interact with the system by using this count, use PageHighMemZone(). > This counter is usually used to calculate the available memory for an > kernel allocation and pages on the highmem zone cannot be available > for an kernel allocation. > 4. Otherwise, use PageHighMemZone(). It's safe since it's implementation > is just copy of the previous PageHighMem() implementation and won't > be changed. hm, this won't improve maintainability :( - Everyone will need to remember when to use PageHighMem() and when to use PageHighMemZone(). If they get it wrong, they're unlikely to notice any problem in their runtime testing, correct? - New code will pop up which gets it wrong and nobody will notice for a long time. So I guess we need to be pretty confident that the series "mm/cma: manage the memory of the CMA area by using the ZONE_MOVABLE" will be useful and merged before proceeding with this, yes? On the other hand, this whole series is a no-op until [10/10] (correct?) so it can be effectively reverted with a single line change, with later cleanups which revert the other 9 patches. So I think I'd like to take another look at "mm/cma: manage the memory of the CMA area by using the ZONE_MOVABLE" before figuring out what to do here. Mainly to answer the question "is the new feature valuable enough to justify the maintainability impact". So please do take some care in explaining the end-user benefit when preparing the new version of that patchset.
2020년 4월 30일 (목) 오전 10:47, Andrew Morton <akpm@linux-foundation.org>님이 작성: > > On Wed, 29 Apr 2020 12:26:33 +0900 js1304@gmail.com wrote: > > > From: Joonsoo Kim <iamjoonsoo.kim@lge.com> > > > > Changes on v2 > > - add "acked-by", "reviewed-by" tags > > - replace PageHighMem() with use open-code, instead of using > > new PageHighMemZone() macro. Related file is "include/linux/migrate.h" > > > > Hello, > > > > This patchset separates two use cases of PageHighMem() by introducing > > PageHighMemZone() macro. And, it changes the implementation of > > PageHighMem() to reflect the actual meaning of this macro. This patchset > > is a preparation step for the patchset, > > "mm/cma: manage the memory of the CMA area by using the ZONE_MOVABLE" [1]. > > > > PageHighMem() is used for two different cases. One is to check if there > > is a direct mapping for this page or not. The other is to check the > > zone of this page, that is, weather it is the highmem type zone or not. > > > > Until now, both the cases are the perfectly same thing. So, implementation > > of the PageHighMem() uses the one case that checks if the zone of the page > > is the highmem type zone or not. > > > > "#define PageHighMem(__p) is_highmem_idx(page_zonenum(__p))" > > > > ZONE_MOVABLE is special. It is considered as normal type zone on > > !CONFIG_HIGHMEM, but, it is considered as highmem type zone > > on CONFIG_HIGHMEM. Let's focus on later case. In later case, all pages > > on the ZONE_MOVABLE has no direct mapping until now. > > > > However, following patchset > > "mm/cma: manage the memory of the CMA area by using the ZONE_MOVABLE" > > , which is once merged and reverted, will be tried again and will break > > this assumption that all pages on the ZONE_MOVABLE has no direct mapping. > > Hence, the ZONE_MOVABLE which is considered as highmem type zone could > > have the both types of pages, direct mapped and not. Since > > the ZONE_MOVABLE could have both type of pages, __GFP_HIGHMEM is still > > required to allocate the memory from it. And, we conservatively need to > > consider the ZONE_MOVABLE as highmem type zone. > > > > Even in this situation, PageHighMem() for the pages on the ZONE_MOVABLE > > when it is called for checking the direct mapping should return correct > > result. Current implementation of PageHighMem() just returns TRUE > > if the zone of the page is on a highmem type zone. So, it could be wrong > > if the page on the MOVABLE_ZONE is actually direct mapped. > > > > To solve this potential problem, this patch introduces a new > > PageHighMemZone() macro. In following patches, two use cases of > > PageHighMem() are separated by calling proper macro, PageHighMem() and > > PageHighMemZone(). Then, implementation of PageHighMem() will be changed > > as just checking if the direct mapping exists or not, regardless of > > the zone of the page. > > > > Note that there are some rules to determine the proper macro. > > > > 1. If PageHighMem() is called for checking if the direct mapping exists > > or not, use PageHighMem(). > > 2. If PageHighMem() is used to predict the previous gfp_flags for > > this page, use PageHighMemZone(). The zone of the page is related to > > the gfp_flags. > > 3. If purpose of calling PageHighMem() is to count highmem page and > > to interact with the system by using this count, use PageHighMemZone(). > > This counter is usually used to calculate the available memory for an > > kernel allocation and pages on the highmem zone cannot be available > > for an kernel allocation. > > 4. Otherwise, use PageHighMemZone(). It's safe since it's implementation > > is just copy of the previous PageHighMem() implementation and won't > > be changed. > > hm, this won't improve maintainability :( > > - Everyone will need to remember when to use PageHighMem() and when > to use PageHighMemZone(). If they get it wrong, they're unlikely to > notice any problem in their runtime testing, correct? > > - New code will pop up which gets it wrong and nobody will notice for > a long time. Hmm... I think that it's not that hard to decide correct macro. If we rename PageHighMem() with PageDirectMapped(), they, PageDirectMapped() and PageHighMemZone(), are self-explanation macro. There would be no confusion to use. > So I guess we need to be pretty confident that the series "mm/cma: > manage the memory of the CMA area by using the ZONE_MOVABLE" will be > useful and merged before proceeding with this, yes? Yes and my assumption is that we (MM) have agreed with usefulness of CMA series. > On the other hand, this whole series is a no-op until [10/10] > (correct?) so it can be effectively reverted with a single line change, Correct! > with later cleanups which revert the other 9 patches. > > So I think I'd like to take another look at "mm/cma: manage the memory > of the CMA area by using the ZONE_MOVABLE" before figuring out what to > do here. Mainly to answer the question "is the new feature valuable > enough to justify the maintainability impact". So please do take some > care in explaining the end-user benefit when preparing the new version > of that patchset. So, do you mean to send the new version of CMA patchset with more explanation before merging this patchset? If yes, I can do. But, I'm not sure that it's worth doing. Problems of CMA are still not solved although the utilization problem will be partially solved by Roman's "mm,page_alloc,cma: conditionally prefer cma pageblocks for movable allocations" patch in this (v5.7) release. Rationale that we agree with CMA patchset is still remained. Anyway, if you mean that, I will send the CMA patchset with more explanation. Thanks.
On Fri, May 01, 2020 at 07:52:35PM +0900, Joonsoo Kim wrote: > > - New code will pop up which gets it wrong and nobody will notice for > > a long time. > > Hmm... I think that it's not that hard to decide correct macro. If we rename > PageHighMem() with PageDirectMapped(), they, PageDirectMapped() and > PageHighMemZone(), are self-explanation macro. There would be no > confusion to use. What confuses me is why we even need PageHighMemZone - mostly code should not care about particular zones. Maybe just open coding PageHighMemZone makes more sense - it is a little more cumersome, but at least it makes it explicit what we check for. I already sent you an incremental diff for one obvious place, but maybe we need to look through the remaining ones if we can kill them or open code them in an obvious way.
2020년 5월 1일 (금) 오후 7:55, Christoph Hellwig <hch@infradead.org>님이 작성: > > On Fri, May 01, 2020 at 07:52:35PM +0900, Joonsoo Kim wrote: > > > - New code will pop up which gets it wrong and nobody will notice for > > > a long time. > > > > Hmm... I think that it's not that hard to decide correct macro. If we rename > > PageHighMem() with PageDirectMapped(), they, PageDirectMapped() and > > PageHighMemZone(), are self-explanation macro. There would be no > > confusion to use. > > What confuses me is why we even need PageHighMemZone - mostly code > should not care about particular zones. Maybe just open coding > PageHighMemZone makes more sense - it is a little more cumersome, but > at least it makes it explicit what we check for. I already sent you > an incremental diff for one obvious place, but maybe we need to look > through the remaining ones if we can kill them or open code them in an > obvious way. I think that PageHighMemZone() is long and complicated enough to have a macro. PageHighMemZone(page) = is_highmem_idx(zone_idx(page_zone(page)) Instead of open-code, how about changing the style of macro like as page_from_highmem()? What PageHighMemZone() represent is derivated attribute from the page so PageXXX() style may not be appropriate. Thanks.
On Fri, May 01, 2020 at 09:15:30PM +0900, Joonsoo Kim wrote: > I think that PageHighMemZone() is long and complicated enough to have > a macro. It is. But then again it also shouldn't really be used by anything but MM internals. > > PageHighMemZone(page) = is_highmem_idx(zone_idx(page_zone(page)) > > Instead of open-code, how about changing the style of macro like as > page_from_highmem()? What PageHighMemZone() represent is derivated > attribute from the page so PageXXX() style may not be appropriate. Maybe page_is_highmem_zone() with a big kerneldoc comment explaining the use case? Bonus points of killing enough users that it can be in mm/internal.h.
2020년 5월 1일 (금) 오후 9:34, Christoph Hellwig <hch@infradead.org>님이 작성: > > On Fri, May 01, 2020 at 09:15:30PM +0900, Joonsoo Kim wrote: > > I think that PageHighMemZone() is long and complicated enough to have > > a macro. > > It is. But then again it also shouldn't really be used by anything > but MM internals. I'm not sure that we can make it MM internal but I will try. > > > > PageHighMemZone(page) = is_highmem_idx(zone_idx(page_zone(page)) > > > > Instead of open-code, how about changing the style of macro like as > > page_from_highmem()? What PageHighMemZone() represent is derivated > > attribute from the page so PageXXX() style may not be appropriate. > > Maybe page_is_highmem_zone() with a big kerneldoc comment explaining > the use case? Bonus points of killing enough users that it can be > in mm/internal.h. I will try to kill page_is_highmem_zone() as much as possible in next version. Thanks.
From: Joonsoo Kim <iamjoonsoo.kim@lge.com> Changes on v2 - add "acked-by", "reviewed-by" tags - replace PageHighMem() with use open-code, instead of using new PageHighMemZone() macro. Related file is "include/linux/migrate.h" Hello, This patchset separates two use cases of PageHighMem() by introducing PageHighMemZone() macro. And, it changes the implementation of PageHighMem() to reflect the actual meaning of this macro. This patchset is a preparation step for the patchset, "mm/cma: manage the memory of the CMA area by using the ZONE_MOVABLE" [1]. PageHighMem() is used for two different cases. One is to check if there is a direct mapping for this page or not. The other is to check the zone of this page, that is, weather it is the highmem type zone or not. Until now, both the cases are the perfectly same thing. So, implementation of the PageHighMem() uses the one case that checks if the zone of the page is the highmem type zone or not. "#define PageHighMem(__p) is_highmem_idx(page_zonenum(__p))" ZONE_MOVABLE is special. It is considered as normal type zone on !CONFIG_HIGHMEM, but, it is considered as highmem type zone on CONFIG_HIGHMEM. Let's focus on later case. In later case, all pages on the ZONE_MOVABLE has no direct mapping until now. However, following patchset "mm/cma: manage the memory of the CMA area by using the ZONE_MOVABLE" , which is once merged and reverted, will be tried again and will break this assumption that all pages on the ZONE_MOVABLE has no direct mapping. Hence, the ZONE_MOVABLE which is considered as highmem type zone could have the both types of pages, direct mapped and not. Since the ZONE_MOVABLE could have both type of pages, __GFP_HIGHMEM is still required to allocate the memory from it. And, we conservatively need to consider the ZONE_MOVABLE as highmem type zone. Even in this situation, PageHighMem() for the pages on the ZONE_MOVABLE when it is called for checking the direct mapping should return correct result. Current implementation of PageHighMem() just returns TRUE if the zone of the page is on a highmem type zone. So, it could be wrong if the page on the MOVABLE_ZONE is actually direct mapped. To solve this potential problem, this patch introduces a new PageHighMemZone() macro. In following patches, two use cases of PageHighMem() are separated by calling proper macro, PageHighMem() and PageHighMemZone(). Then, implementation of PageHighMem() will be changed as just checking if the direct mapping exists or not, regardless of the zone of the page. Note that there are some rules to determine the proper macro. 1. If PageHighMem() is called for checking if the direct mapping exists or not, use PageHighMem(). 2. If PageHighMem() is used to predict the previous gfp_flags for this page, use PageHighMemZone(). The zone of the page is related to the gfp_flags. 3. If purpose of calling PageHighMem() is to count highmem page and to interact with the system by using this count, use PageHighMemZone(). This counter is usually used to calculate the available memory for an kernel allocation and pages on the highmem zone cannot be available for an kernel allocation. 4. Otherwise, use PageHighMemZone(). It's safe since it's implementation is just copy of the previous PageHighMem() implementation and won't be changed. My final plan is to change the name, PageHighMem() to PageNoDirectMapped() or something else in order to represent proper meaning. This patchset is based on next-20200428 and you can find the full patchset on the following link. https://github.com/JoonsooKim/linux/tree/page_highmem-cleanup-v2.00-next-20200428 Thanks. [1]: https://lore.kernel.org/linux-mm/1512114786-5085-1-git-send-email-iamjoonsoo.kim@lge.com Joonsoo Kim (10): mm/page-flags: introduce PageHighMemZone() drm/ttm: separate PageHighMem() and PageHighMemZone() use case kexec: separate PageHighMem() and PageHighMemZone() use case power: separate PageHighMem() and PageHighMemZone() use case mm/gup: separate PageHighMem() and PageHighMemZone() use case mm/hugetlb: separate PageHighMem() and PageHighMemZone() use case mm: separate PageHighMem() and PageHighMemZone() use case mm/page_alloc: correct the use of is_highmem_idx() mm/migrate: replace PageHighMem() with open-code mm/page-flags: change the implementation of the PageHighMem() drivers/gpu/drm/ttm/ttm_memory.c | 4 ++-- drivers/gpu/drm/ttm/ttm_page_alloc.c | 2 +- drivers/gpu/drm/ttm/ttm_page_alloc_dma.c | 2 +- drivers/gpu/drm/ttm/ttm_tt.c | 2 +- include/linux/migrate.h | 4 +++- include/linux/page-flags.h | 10 +++++++++- kernel/kexec_core.c | 2 +- kernel/power/snapshot.c | 12 ++++++------ mm/gup.c | 2 +- mm/hugetlb.c | 2 +- mm/memory_hotplug.c | 2 +- mm/page_alloc.c | 4 ++-- 12 files changed, 29 insertions(+), 19 deletions(-)