| Message ID | 20200915125947.26204-1-songmuchun@bytedance.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | mm/hugetlb: Free some vmemmap pages of hugetlb page | expand |
On Tue, Sep 15, 2020 at 08:59:23PM +0800, Muchun Song wrote: > This patch series will free some vmemmap pages(struct page structures) > associated with each hugetlbpage when preallocated to save memory. It would be lovely to be able to do this. Unfortunately, it's completely impossible right now. Consider, for example, get_user_pages() called on the fifth page of a hugetlb page. I've spent a lot of time thinking about this, and there's a lot of work that needs to happen before we can do this, mostly in device drivers. Do you want to help? It's a multi-year project.
On 9/15/20 7:32 AM, Matthew Wilcox wrote: > On Tue, Sep 15, 2020 at 08:59:23PM +0800, Muchun Song wrote: >> This patch series will free some vmemmap pages(struct page structures) >> associated with each hugetlbpage when preallocated to save memory. > It would be lovely to be able to do this. Unfortunately, it's completely > impossible right now. Consider, for example, get_user_pages() called > on the fifth page of a hugetlb page. Yeah, exactly. Does this series survive the in-kernel selftests/? If so, sounds like we need to add a new selftest.
On Tue, Sep 15, 2020 at 10:32 PM Matthew Wilcox <willy@infradead.org> wrote: > > On Tue, Sep 15, 2020 at 08:59:23PM +0800, Muchun Song wrote: > > This patch series will free some vmemmap pages(struct page structures) > > associated with each hugetlbpage when preallocated to save memory. > > It would be lovely to be able to do this. Unfortunately, it's completely > impossible right now. Consider, for example, get_user_pages() called > on the fifth page of a hugetlb page. Can you elaborate on the problem? Thanks so much. > > I've spent a lot of time thinking about this, and there's a lot of work > that needs to happen before we can do this, mostly in device drivers. > Do you want to help? It's a multi-year project. Welcome, we can work together.
On Tue, Sep 15, 2020 at 11:28:01PM +0800, Muchun Song wrote: > On Tue, Sep 15, 2020 at 10:32 PM Matthew Wilcox <willy@infradead.org> wrote: > > > > On Tue, Sep 15, 2020 at 08:59:23PM +0800, Muchun Song wrote: > > > This patch series will free some vmemmap pages(struct page structures) > > > associated with each hugetlbpage when preallocated to save memory. > > > > It would be lovely to be able to do this. Unfortunately, it's completely > > impossible right now. Consider, for example, get_user_pages() called > > on the fifth page of a hugetlb page. > > Can you elaborate on the problem? Thanks so much. OK, let's say you want to do a 2kB I/O to offset 0x5000 of a 2MB page on a 4kB base page system. Today, that results in a bio_vec containing {head+5, 0, 0x800}. Then we call page_to_phys() on that (head+5) struct page to get the physical address of the I/O, and we turn it into a struct scatterlist, which similarly has a reference to the page (head+5). If you're returning page (head+1) from get_user_pages(), all the calculations will be wrong and you'll do DMA to the wrong address. What needs to happen is a conversion to get_user_bvecs(). That way we can return a bvec which is {head, 0x5000, 0x800} and the calculations will work. So that's going to involve touching a lot of device drivers. Christoph has a start on it here: http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/gup-bvec Overlapping with that is a desire to change the biovec so that it only stores {phys_addr, length} rather than {page, offset, length}. We're also going to have to rework the scatterlist so that it doesn't carry a struct page either.
On Tue, Sep 15, 2020 at 11:42 PM Matthew Wilcox <willy@infradead.org> wrote: > > On Tue, Sep 15, 2020 at 11:28:01PM +0800, Muchun Song wrote: > > On Tue, Sep 15, 2020 at 10:32 PM Matthew Wilcox <willy@infradead.org> wrote: > > > > > > On Tue, Sep 15, 2020 at 08:59:23PM +0800, Muchun Song wrote: > > > > This patch series will free some vmemmap pages(struct page structures) > > > > associated with each hugetlbpage when preallocated to save memory. > > > > > > It would be lovely to be able to do this. Unfortunately, it's completely > > > impossible right now. Consider, for example, get_user_pages() called > > > on the fifth page of a hugetlb page. > > > > Can you elaborate on the problem? Thanks so much. > > OK, let's say you want to do a 2kB I/O to offset 0x5000 of a 2MB page > on a 4kB base page system. Today, that results in a bio_vec containing > {head+5, 0, 0x800}. Then we call page_to_phys() on that (head+5) struct > page to get the physical address of the I/O, and we turn it into a struct > scatterlist, which similarly has a reference to the page (head+5). As I know, in this case, the get_user_pages() will get a reference to the head page (head+0) before returning such that the hugetlb page can not be freed. Although get_user_pages() returns the page (head+5) and the scatterlist has a reference to the page (head+5), this patch series can handle this situation. I can not figure out where the problem is. What I missed? Thanks. > > If you're returning page (head+1) from get_user_pages(), all the > calculations will be wrong and you'll do DMA to the wrong address. > > What needs to happen is a conversion to get_user_bvecs(). That way we > can return a bvec which is {head, 0x5000, 0x800} and the calculations > will work. So that's going to involve touching a lot of device drivers. > Christoph has a start on it here: > http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/gup-bvec > > Overlapping with that is a desire to change the biovec so that it > only stores {phys_addr, length} rather than {page, offset, length}. > We're also going to have to rework the scatterlist so that it doesn't > carry a struct page either.
On Wed, Sep 16, 2020 at 01:32:46AM +0800, Muchun Song wrote: > On Tue, Sep 15, 2020 at 11:42 PM Matthew Wilcox <willy@infradead.org> wrote: > > > > On Tue, Sep 15, 2020 at 11:28:01PM +0800, Muchun Song wrote: > > > On Tue, Sep 15, 2020 at 10:32 PM Matthew Wilcox <willy@infradead.org> wrote: > > > > > > > > On Tue, Sep 15, 2020 at 08:59:23PM +0800, Muchun Song wrote: > > > > > This patch series will free some vmemmap pages(struct page structures) > > > > > associated with each hugetlbpage when preallocated to save memory. > > > > > > > > It would be lovely to be able to do this. Unfortunately, it's completely > > > > impossible right now. Consider, for example, get_user_pages() called > > > > on the fifth page of a hugetlb page. > > > > > > Can you elaborate on the problem? Thanks so much. > > > > OK, let's say you want to do a 2kB I/O to offset 0x5000 of a 2MB page > > on a 4kB base page system. Today, that results in a bio_vec containing > > {head+5, 0, 0x800}. Then we call page_to_phys() on that (head+5) struct > > page to get the physical address of the I/O, and we turn it into a struct > > scatterlist, which similarly has a reference to the page (head+5). > > As I know, in this case, the get_user_pages() will get a reference > to the head page (head+0) before returning such that the hugetlb > page can not be freed. Although get_user_pages() returns the > page (head+5) and the scatterlist has a reference to the page > (head+5), this patch series can handle this situation. I can not > figure out where the problem is. What I missed? Thanks. You freed pages 4-511 from the vmemmap so they could be used for something else. Page 5 isn't there any more. So if you return head+5, then when we complete the I/O, we'll look for the compound_head() of head+5 and we won't find head.
On Wed, Sep 16, 2020 at 1:39 AM Matthew Wilcox <willy@infradead.org> wrote: > > On Wed, Sep 16, 2020 at 01:32:46AM +0800, Muchun Song wrote: > > On Tue, Sep 15, 2020 at 11:42 PM Matthew Wilcox <willy@infradead.org> wrote: > > > > > > On Tue, Sep 15, 2020 at 11:28:01PM +0800, Muchun Song wrote: > > > > On Tue, Sep 15, 2020 at 10:32 PM Matthew Wilcox <willy@infradead.org> wrote: > > > > > > > > > > On Tue, Sep 15, 2020 at 08:59:23PM +0800, Muchun Song wrote: > > > > > > This patch series will free some vmemmap pages(struct page structures) > > > > > > associated with each hugetlbpage when preallocated to save memory. > > > > > > > > > > It would be lovely to be able to do this. Unfortunately, it's completely > > > > > impossible right now. Consider, for example, get_user_pages() called > > > > > on the fifth page of a hugetlb page. > > > > > > > > Can you elaborate on the problem? Thanks so much. > > > > > > OK, let's say you want to do a 2kB I/O to offset 0x5000 of a 2MB page > > > on a 4kB base page system. Today, that results in a bio_vec containing > > > {head+5, 0, 0x800}. Then we call page_to_phys() on that (head+5) struct > > > page to get the physical address of the I/O, and we turn it into a struct > > > scatterlist, which similarly has a reference to the page (head+5). > > > > As I know, in this case, the get_user_pages() will get a reference > > to the head page (head+0) before returning such that the hugetlb > > page can not be freed. Although get_user_pages() returns the > > page (head+5) and the scatterlist has a reference to the page > > (head+5), this patch series can handle this situation. I can not > > figure out where the problem is. What I missed? Thanks. > > You freed pages 4-511 from the vmemmap so they could be used for > something else. Page 5 isn't there any more. So if you return head+5, > then when we complete the I/O, we'll look for the compound_head() of > head+5 and we won't find head. > We do not free pages 4-511 from the vmemmap. Actually, we only free pages 128-511 from the vmemmap. The 512 struct pages occupy 8 pages of physical memory. We only free 6 physical page frames to the buddy. But we will create a new mapping just like below. The virtual address of the freed pages will remap to the second page frame. So the second page frame is reused. When a hugetlbpage is preallocated, we can change the mapping to bellow. hugetlbpage struct page(8 pages) page frame(8 pages) +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ | | | 0 | -------------> | 0 | | | | 1 | -------------> | 1 | | | | 2 | -------------> +-----------+ | | | 3 | -----------------^ ^ ^ ^ ^ | | | 4 | -------------------+ | | | | 2M | | 5 | ---------------------+ | | | | | 6 | -----------------------+ | | | | 7 | -------------------------+ | | +-----------+ | | | | +-----------+ As you can see, we reuse the first tail page.
On Wed, Sep 16, 2020 at 02:03:15AM +0800, Muchun Song wrote: > On Wed, Sep 16, 2020 at 1:39 AM Matthew Wilcox <willy@infradead.org> wrote: > > > > On Wed, Sep 16, 2020 at 01:32:46AM +0800, Muchun Song wrote: > > > On Tue, Sep 15, 2020 at 11:42 PM Matthew Wilcox <willy@infradead.org> wrote: > > > > > > > > On Tue, Sep 15, 2020 at 11:28:01PM +0800, Muchun Song wrote: > > > > > On Tue, Sep 15, 2020 at 10:32 PM Matthew Wilcox <willy@infradead.org> wrote: > > > > > > > > > > > > On Tue, Sep 15, 2020 at 08:59:23PM +0800, Muchun Song wrote: > > > > > > > This patch series will free some vmemmap pages(struct page structures) > > > > > > > associated with each hugetlbpage when preallocated to save memory. > > > > > > > > > > > > It would be lovely to be able to do this. Unfortunately, it's completely > > > > > > impossible right now. Consider, for example, get_user_pages() called > > > > > > on the fifth page of a hugetlb page. > > > > > > > > > > Can you elaborate on the problem? Thanks so much. > > > > > > > > OK, let's say you want to do a 2kB I/O to offset 0x5000 of a 2MB page > > > > on a 4kB base page system. Today, that results in a bio_vec containing > > > > {head+5, 0, 0x800}. Then we call page_to_phys() on that (head+5) struct > > > > page to get the physical address of the I/O, and we turn it into a struct > > > > scatterlist, which similarly has a reference to the page (head+5). > > > > > > As I know, in this case, the get_user_pages() will get a reference > > > to the head page (head+0) before returning such that the hugetlb > > > page can not be freed. Although get_user_pages() returns the > > > page (head+5) and the scatterlist has a reference to the page > > > (head+5), this patch series can handle this situation. I can not > > > figure out where the problem is. What I missed? Thanks. > > > > You freed pages 4-511 from the vmemmap so they could be used for > > something else. Page 5 isn't there any more. So if you return head+5, > > then when we complete the I/O, we'll look for the compound_head() of > > head+5 and we won't find head. > > We do not free pages 4-511 from the vmemmap. Actually, we only > free pages 128-511 from the vmemmap. > > The 512 struct pages occupy 8 pages of physical memory. We only > free 6 physical page frames to the buddy. But we will create a new > mapping just like below. The virtual address of the freed pages will > remap to the second page frame. So the second page frame is > reused. Oh! I get what you're doing now. For the vmemmap case, you free the last N-2 physical pages but map the second physical page multiple times. So for the 512 pages case, we see pages: abcdefgh | ijklmnop | ijklmnop | ijklmnop | ijklmnop | ijklmnop | ijklmnop ... Huh. I think that might work, except for PageHWPoison. I'll go back to your patch series and look at that some more.
On Wed, Sep 16, 2020 at 2:15 AM Matthew Wilcox <willy@infradead.org> wrote: > > On Wed, Sep 16, 2020 at 02:03:15AM +0800, Muchun Song wrote: > > On Wed, Sep 16, 2020 at 1:39 AM Matthew Wilcox <willy@infradead.org> wrote: > > > > > > On Wed, Sep 16, 2020 at 01:32:46AM +0800, Muchun Song wrote: > > > > On Tue, Sep 15, 2020 at 11:42 PM Matthew Wilcox <willy@infradead.org> wrote: > > > > > > > > > > On Tue, Sep 15, 2020 at 11:28:01PM +0800, Muchun Song wrote: > > > > > > On Tue, Sep 15, 2020 at 10:32 PM Matthew Wilcox <willy@infradead.org> wrote: > > > > > > > > > > > > > > On Tue, Sep 15, 2020 at 08:59:23PM +0800, Muchun Song wrote: > > > > > > > > This patch series will free some vmemmap pages(struct page structures) > > > > > > > > associated with each hugetlbpage when preallocated to save memory. > > > > > > > > > > > > > > It would be lovely to be able to do this. Unfortunately, it's completely > > > > > > > impossible right now. Consider, for example, get_user_pages() called > > > > > > > on the fifth page of a hugetlb page. > > > > > > > > > > > > Can you elaborate on the problem? Thanks so much. > > > > > > > > > > OK, let's say you want to do a 2kB I/O to offset 0x5000 of a 2MB page > > > > > on a 4kB base page system. Today, that results in a bio_vec containing > > > > > {head+5, 0, 0x800}. Then we call page_to_phys() on that (head+5) struct > > > > > page to get the physical address of the I/O, and we turn it into a struct > > > > > scatterlist, which similarly has a reference to the page (head+5). > > > > > > > > As I know, in this case, the get_user_pages() will get a reference > > > > to the head page (head+0) before returning such that the hugetlb > > > > page can not be freed. Although get_user_pages() returns the > > > > page (head+5) and the scatterlist has a reference to the page > > > > (head+5), this patch series can handle this situation. I can not > > > > figure out where the problem is. What I missed? Thanks. > > > > > > You freed pages 4-511 from the vmemmap so they could be used for > > > something else. Page 5 isn't there any more. So if you return head+5, > > > then when we complete the I/O, we'll look for the compound_head() of > > > head+5 and we won't find head. > > > > We do not free pages 4-511 from the vmemmap. Actually, we only > > free pages 128-511 from the vmemmap. > > > > The 512 struct pages occupy 8 pages of physical memory. We only > > free 6 physical page frames to the buddy. But we will create a new > > mapping just like below. The virtual address of the freed pages will > > remap to the second page frame. So the second page frame is > > reused. > > Oh! I get what you're doing now. > > For the vmemmap case, you free the last N-2 physical pages but map the > second physical page multiple times. So for the 512 pages case, we > see pages: > > abcdefgh | ijklmnop | ijklmnop | ijklmnop | ijklmnop | ijklmnop | ijklmnop ... Yeah, great. You are right. > > Huh. I think that might work, except for PageHWPoison. I'll go back > to your patch series and look at that some more. > The PageHWPoison also is considered in the patch series. Looking forward to your suggestions. Thanks.
On 9/15/20 5:59 AM, Muchun Song wrote: > Hi all, > > This patch series will free some vmemmap pages(struct page structures) > associated with each hugetlbpage when preallocated to save memory. ... > The mapping of the first page(index 0) and the second page(index 1) is > unchanged. The remaining 6 pages are all mapped to the same page(index > 1). So we only need 2 pages for vmemmap area and free 6 pages to the > buddy system to save memory. Why we can do this? Because the content > of the remaining 7 pages are usually same except the first page. > > When a hugetlbpage is freed to the buddy system, we should allocate 6 > pages for vmemmap pages and restore the previous mapping relationship. > > If we uses the 1G hugetlbpage, we can save 4095 pages. This is a very > substantial gain. On our server, run some SPDK applications which will > use 300GB hugetlbpage. With this feature enabled, we can save 4797MB > memory. At a high level this seems like a reasonable optimization for hugetlb pages. It is possible because hugetlb pages are 'special' and mostly handled differently than pages in normal mm paths. The majority of the new code is hugetlb specific, so it should not be of too much concern for the general mm code paths. I'll start looking closer at the series. However, if someone has high level concerns please let us know. The only 'potential' conflict I am aware of is discussion about support of double mapping hugetlb pages. https://lists.gnu.org/archive/html/qemu-devel/2017-03/msg02407.html More care/coordination would be needed to support double mapping with this new option. However, this series provides a boot option to disable freeing of unneeded page structs.
On Tue, Sep 29, 2020 at 02:58:18PM -0700, Mike Kravetz wrote: > On 9/15/20 5:59 AM, Muchun Song wrote: > > Hi all, > > > > This patch series will free some vmemmap pages(struct page structures) > > associated with each hugetlbpage when preallocated to save memory. > ... > > The mapping of the first page(index 0) and the second page(index 1) is > > unchanged. The remaining 6 pages are all mapped to the same page(index > > 1). So we only need 2 pages for vmemmap area and free 6 pages to the > > buddy system to save memory. Why we can do this? Because the content > > of the remaining 7 pages are usually same except the first page. > > > > When a hugetlbpage is freed to the buddy system, we should allocate 6 > > pages for vmemmap pages and restore the previous mapping relationship. > > > > If we uses the 1G hugetlbpage, we can save 4095 pages. This is a very > > substantial gain. On our server, run some SPDK applications which will > > use 300GB hugetlbpage. With this feature enabled, we can save 4797MB > > memory. > > At a high level this seems like a reasonable optimization for hugetlb > pages. It is possible because hugetlb pages are 'special' and mostly > handled differently than pages in normal mm paths. > > The majority of the new code is hugetlb specific, so it should not be > of too much concern for the general mm code paths. I'll start looking > closer at the series. However, if someone has high level concerns please > let us know. The only 'potential' conflict I am aware of is discussion > about support of double mapping hugetlb pages. Not on x86, but architectures which have dcache coherency issues sometimes use PG_arch_1 on the subpages. I think it would be wise to map pages 1-7 read-only to catch this, as well as any future change which causes subpage bits to get set.
On 9/29/20 2:58 PM, Mike Kravetz wrote: > On 9/15/20 5:59 AM, Muchun Song wrote: >> Hi all, >> >> This patch series will free some vmemmap pages(struct page structures) >> associated with each hugetlbpage when preallocated to save memory. > ... >> The mapping of the first page(index 0) and the second page(index 1) is >> unchanged. The remaining 6 pages are all mapped to the same page(index >> 1). So we only need 2 pages for vmemmap area and free 6 pages to the >> buddy system to save memory. Why we can do this? Because the content >> of the remaining 7 pages are usually same except the first page. >> >> When a hugetlbpage is freed to the buddy system, we should allocate 6 >> pages for vmemmap pages and restore the previous mapping relationship. >> >> If we uses the 1G hugetlbpage, we can save 4095 pages. This is a very >> substantial gain. On our server, run some SPDK applications which will >> use 300GB hugetlbpage. With this feature enabled, we can save 4797MB >> memory. I had a hard time going through the patch series as it is currently structured, and instead examined all the code together. Muchun put in much effort and the code does reduce memory usage. - For 2MB hugetlb pages, we save 5 pages of struct pages - For 1GB hugetlb pages, we save 4086 pages of struct pages Code is even in pace to handle poisoned pages, although I have not looked at this closely. The code survives the libhugetlbfs and ltp huge page tests. To date, nobody has asked the important question "Is the added complexity worth the memory savings?". I suppose it all depends on one's use case. Obviously, the savings are more significant when one uses 1G huge pages but that may not be the common case today. > At a high level this seems like a reasonable optimization for hugetlb > pages. It is possible because hugetlb pages are 'special' and mostly > handled differently than pages in normal mm paths. Such an optimization only makes sense for something like hugetlb pages. One reason is the 'special' nature of hugetlbfs as stated above. The other is that this optimization mostly makes sense for huge pages that are created once and stick around for a long time. hugetlb pool pages are a perfect example. This is because manipulation of struct page mappings is done when a huge page is created or destroyed. > The majority of the new code is hugetlb specific, so it should not be > of too much concern for the general mm code paths. It is true that much of the code in this series was put in hugetlb.c. However, I would argue that there is a bunch of code that only deals with remapping the memmap which should more generic and added to sparse-vmemmap.c. This would at least allow for easier reuse. Before Muchun and myself put more effort into this series, I would really like to get feedback on the whether or not this should move forward. Specifically, is the memory savings worth added complexity? Is the removing of struct pages going to come back and cause issues for future features?
On Thu, Oct 8, 2020 at 5:15 AM Mike Kravetz <mike.kravetz@oracle.com> wrote: > > On 9/29/20 2:58 PM, Mike Kravetz wrote: > > On 9/15/20 5:59 AM, Muchun Song wrote: > >> Hi all, > >> > >> This patch series will free some vmemmap pages(struct page structures) > >> associated with each hugetlbpage when preallocated to save memory. > > ... > >> The mapping of the first page(index 0) and the second page(index 1) is > >> unchanged. The remaining 6 pages are all mapped to the same page(index > >> 1). So we only need 2 pages for vmemmap area and free 6 pages to the > >> buddy system to save memory. Why we can do this? Because the content > >> of the remaining 7 pages are usually same except the first page. > >> > >> When a hugetlbpage is freed to the buddy system, we should allocate 6 > >> pages for vmemmap pages and restore the previous mapping relationship. > >> > >> If we uses the 1G hugetlbpage, we can save 4095 pages. This is a very > >> substantial gain. On our server, run some SPDK applications which will > >> use 300GB hugetlbpage. With this feature enabled, we can save 4797MB > >> memory. > > I had a hard time going through the patch series as it is currently > structured, and instead examined all the code together. Muchun put in > much effort and the code does reduce memory usage. > - For 2MB hugetlb pages, we save 5 pages of struct pages > - For 1GB hugetlb pages, we save 4086 pages of struct pages > > Code is even in pace to handle poisoned pages, although I have not looked > at this closely. The code survives the libhugetlbfs and ltp huge page tests. > > To date, nobody has asked the important question "Is the added complexity > worth the memory savings?". I suppose it all depends on one's use case. > Obviously, the savings are more significant when one uses 1G huge pages but > that may not be the common case today. > > > At a high level this seems like a reasonable optimization for hugetlb > > pages. It is possible because hugetlb pages are 'special' and mostly > > handled differently than pages in normal mm paths. > > Such an optimization only makes sense for something like hugetlb pages. One > reason is the 'special' nature of hugetlbfs as stated above. The other is > that this optimization mostly makes sense for huge pages that are created > once and stick around for a long time. hugetlb pool pages are a perfect > example. This is because manipulation of struct page mappings is done when > a huge page is created or destroyed. Yeah, in our cloud server, we have some application scenarios(e.g. SPDK, DPDK, QEMU and jemalloc). These applications may use a lot of hugetlb pages. > > > The majority of the new code is hugetlb specific, so it should not be > > of too much concern for the general mm code paths. > > It is true that much of the code in this series was put in hugetlb.c. However, > I would argue that there is a bunch of code that only deals with remapping > the memmap which should more generic and added to sparse-vmemmap.c. This > would at least allow for easier reuse. I agree with you. > > Before Muchun and myself put more effort into this series, I would really > like to get feedback on the whether or not this should move forward. > Specifically, is the memory savings worth added complexity? Is the removing > of struct pages going to come back and cause issues for future features? Some users do need this optimization to save memory. But if some users do not need this optimization, they also can disable it by using a kernel boot parameter 'hugetlb_free_vmemmap=off' or not configuring CONFIG_HUGETLB_PAGE_FREE_VMEMMAP. I have no idea about "cause issues for future features". Is there any feature ongoing or planned?
Hi all, This patch series will free some vmemmap pages(struct page structures) associated with each hugetlbpage when preallocated to save memory. Nowadays we track the status of physical page frames using `struct page` arranged in one or more arrays. And here exists one-to-one mapping between the physical page frame and the corresponding `struct page`. The hugetlbpage support is built on top of multiple page size support that is provided by most modern architectures. For example, x86 CPUs normally support 4K and 2M (1G if architecturally supported) page sizes. Every hugetlbpage has more than one `struct page`. The 2M hugetlbpage has 512 `struct page` and 1G hugetlbpage has 4096 `struct page`. But in the core of hugetlbpage only uses the first 4 `struct page` to store metadata associated with each hugetlbpage. The rest of the `struct page` are usually read the compound_head field which are all the same value. If we can free some struct page memory to buddy system so that We can save a lot of memory. When the system boot up, every 2M hugetlbpage has 512 `struct page` which is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE). hugetlbpage struct page(8 pages) page frame(8 pages) +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ | | | 0 | -------------> | 0 | | | | 1 | -------------> | 1 | | | | 2 | -------------> | 2 | | | | 3 | -------------> | 3 | | | | 4 | -------------> | 4 | | 2M | | 5 | -------------> | 5 | | | | 6 | -------------> | 6 | | | | 7 | -------------> | 7 | | | +-----------+ +-----------+ | | | | +-----------+ When a hugetlbpage is preallocated, we can change the mapping from above to bellow. hugetlbpage struct page(8 pages) page frame(8 pages) +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ | | | 0 | -------------> | 0 | | | | 1 | -------------> | 1 | | | | 2 | -------------> +-----------+ | | | 3 | -----------------^ ^ ^ ^ ^ | | | 4 | -------------------+ | | | | 2M | | 5 | ---------------------+ | | | | | 6 | -----------------------+ | | | | 7 | -------------------------+ | | +-----------+ | | | | +-----------+ The mapping of the first page(index 0) and the second page(index 1) is unchanged. The remaining 6 pages are all mapped to the same page(index 1). So we only need 2 pages for vmemmap area and free 6 pages to the buddy system to save memory. Why we can do this? Because the content of the remaining 7 pages are usually same except the first page. When a hugetlbpage is freed to the buddy system, we should allocate 6 pages for vmemmap pages and restore the previous mapping relationship. If we uses the 1G hugetlbpage, we can save 4095 pages. This is a very substantial gain. On our server, run some SPDK applications which will use 300GB hugetlbpage. With this feature enabled, we can save 4797MB memory. Muchun Song (24): mm/memory_hotplug: Move bootmem info registration API to bootmem_info.c mm/memory_hotplug: Move {get,put}_page_bootmem() to bootmem_info.c mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP mm/hugetlb: Register bootmem info when CONFIG_HUGETLB_PAGE_FREE_VMEMMAP mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate mm/hugetlb: Introduce pgtable allocation/freeing helpers mm/hugetlb: Add freeing unused vmemmap pages support for x86 mm/bootmem_info: Introduce {free,prepare}_vmemmap_page() x86/mm: Introduce VMEMMAP_SIZE/VMEMMAP_END macro mm/hugetlb: Free the vmemmap pages associated with each hugetlb page mm/hugetlb: Add vmemmap_pmd_huge macro for x86 mm/hugetlb: Defer freeing of hugetlb pages mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb page mm/hugetlb: Introduce remap_huge_page_pmd_vmemmap helper mm/hugetlb: Use PG_slab to indicate split pmd mm/hugetlb: Support freeing vmemmap pages of gigantic page mm/hugetlb: Add a BUILD_BUG_ON to check if struct page size is a power of two mm/hugetlb: Clear PageHWPoison on the non-error memory page mm/hugetlb: Flush work when dissolving hugetlb page mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap mm/hugetlb: Merge pte to huge pmd only for gigantic page mm/hugetlb: Implement vmemmap_pmd_mkhuge macro mm/hugetlb: Gather discrete indexes of tail page mm/hugetlb: Add BUILD_BUG_ON to catch invalid usage of tail struct page .../admin-guide/kernel-parameters.txt | 9 + Documentation/admin-guide/mm/hugetlbpage.rst | 3 + arch/x86/include/asm/hugetlb.h | 20 + arch/x86/include/asm/pgtable_64_types.h | 8 + arch/x86/mm/init_64.c | 5 +- fs/Kconfig | 15 + include/linux/bootmem_info.h | 65 ++ include/linux/hugetlb.h | 64 ++ include/linux/hugetlb_cgroup.h | 15 +- include/linux/memory_hotplug.h | 27 - mm/Makefile | 1 + mm/bootmem_info.c | 125 +++ mm/hugetlb.c | 834 +++++++++++++++++- mm/memory_hotplug.c | 116 --- mm/sparse.c | 1 + 15 files changed, 1143 insertions(+), 165 deletions(-) create mode 100644 include/linux/bootmem_info.h create mode 100644 mm/bootmem_info.c