| Message ID | 20230524153311.3625329-5-dhowells@redhat.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | None | expand |
On 2023/5/24 23:33, David Howells wrote: > Change the page_frag_cache allocator to use multipage folios rather than > groups of pages. This reduces page_frag_free to just a folio_put() or > put_page(). Hi, David put_page() is not used in this patch, perhaps remove it to avoid the confusion? Also, Is there any significant difference between __free_pages() and folio_put()? IOW, what does the 'reduces' part means here? I followed some disscusion about folio before, but have not really understood about real difference between 'multipage folios' and 'groups of pages' yet. Is folio mostly used to avoid the confusion about whether a page is 'headpage of compound page', 'base page' or 'tailpage of compound page'? Or is there any abvious benefit about folio that I missed? > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 306a3d1a0fa6..d7c52a5979cc 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -420,18 +420,13 @@ static inline void *folio_get_private(struct folio *folio) > } > > struct page_frag_cache { > - void * va; > -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) > - __u16 offset; > - __u16 size; > -#else > - __u32 offset; > -#endif > + struct folio *folio; > + unsigned int offset; > /* we maintain a pagecount bias, so that we dont dirty cache line > * containing page->_refcount every time we allocate a fragment. > */ > - unsigned int pagecnt_bias; > - bool pfmemalloc; > + unsigned int pagecnt_bias; > + bool pfmemalloc; > }; It seems 'va' and 'size' field is used to avoid touching 'stuct page' to avoid possible cache bouncing when there is more frag can be allocated from the page while other frags is freed at the same time before this patch? It might be worth calling that out in the commit log or split it into another patch to make it clearer and easier to review?
Yunsheng Lin <linyunsheng@huawei.com> wrote: > > Change the page_frag_cache allocator to use multipage folios rather than > > groups of pages. This reduces page_frag_free to just a folio_put() or > > put_page(). > > put_page() is not used in this patch, perhaps remove it to avoid > the confusion? Will do if I need to respin the patches. > Also, Is there any significant difference between __free_pages() > and folio_put()? IOW, what does the 'reduces' part means here? I meant that the folio code handles page compounding for us and we don't need to work out how big the page is for ourselves. If you look at __free_pages(), you can see a PageHead() call. folio_put() doesn't need that. > I followed some disscusion about folio before, but have not really > understood about real difference between 'multipage folios' and > 'groups of pages' yet. Is folio mostly used to avoid the confusion > about whether a page is 'headpage of compound page', 'base page' or > 'tailpage of compound page'? Or is there any abvious benefit about > folio that I missed? There is a benefit: a folio pointer always points to the head page and so we never need to do "is this compound? where's the head?" logic to find it. When going from a page pointer, we still have to find the head. Ultimately, the aim is to reduce struct page to a typed pointer to massively reduce the amount of space consumed by mem_map[]. A page struct will then point at a folio or a slab struct or one of a number of different types. But to get to that point, we have to stop a whole lot of things from using page structs, but rather use some other type, such as folio. Eventually, there won't be a need for head pages and tail pages per se - just memory objects of different sizes. > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > > index 306a3d1a0fa6..d7c52a5979cc 100644 > > --- a/include/linux/mm_types.h > > +++ b/include/linux/mm_types.h > > @@ -420,18 +420,13 @@ static inline void *folio_get_private(struct folio *folio) > > } > > > > struct page_frag_cache { > > - void * va; > > -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) > > - __u16 offset; > > - __u16 size; > > -#else > > - __u32 offset; > > -#endif > > + struct folio *folio; > > + unsigned int offset; > > /* we maintain a pagecount bias, so that we dont dirty cache line > > * containing page->_refcount every time we allocate a fragment. > > */ > > - unsigned int pagecnt_bias; > > - bool pfmemalloc; > > + unsigned int pagecnt_bias; > > + bool pfmemalloc; > > }; > > It seems 'va' and 'size' field is used to avoid touching 'stuct page' to > avoid possible cache bouncing when there is more frag can be allocated > from the page while other frags is freed at the same time before this patch? Hmmm... fair point, though va is calculated from the page pointer on most arches without the need to dereference struct page (only arc, m68k and sparc define WANT_PAGE_VIRTUAL). David
Hi, On 26.5.2023 15.47, David Howells wrote: > Yunsheng Lin <linyunsheng@huawei.com> wrote: > >>> Change the page_frag_cache allocator to use multipage folios rather than >>> groups of pages. This reduces page_frag_free to just a folio_put() or >>> put_page(). >> >> put_page() is not used in this patch, perhaps remove it to avoid >> the confusion? > > Will do if I need to respin the patches. > >> Also, Is there any significant difference between __free_pages() >> and folio_put()? IOW, what does the 'reduces' part means here? > > I meant that the folio code handles page compounding for us and we don't need > to work out how big the page is for ourselves. > > If you look at __free_pages(), you can see a PageHead() call. folio_put() > doesn't need that. > >> I followed some disscusion about folio before, but have not really >> understood about real difference between 'multipage folios' and >> 'groups of pages' yet. Is folio mostly used to avoid the confusion >> about whether a page is 'headpage of compound page', 'base page' or >> 'tailpage of compound page'? Or is there any abvious benefit about >> folio that I missed? > > There is a benefit: a folio pointer always points to the head page and so we > never need to do "is this compound? where's the head?" logic to find it. When > going from a page pointer, we still have to find the head. > But page_frag_free() uses folio_put(virt_to_folio(addr)) and virt_to_folio() depends on the compound infrastructure to get the head page and folio. > Ultimately, the aim is to reduce struct page to a typed pointer to massively > reduce the amount of space consumed by mem_map[]. A page struct will then > point at a folio or a slab struct or one of a number of different types. But > to get to that point, we have to stop a whole lot of things from using page > structs, but rather use some other type, such as folio. > > Eventually, there won't be a need for head pages and tail pages per se - just > memory objects of different sizes. > >>> diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h >>> index 306a3d1a0fa6..d7c52a5979cc 100644 >>> --- a/include/linux/mm_types.h >>> +++ b/include/linux/mm_types.h >>> @@ -420,18 +420,13 @@ static inline void *folio_get_private(struct folio *folio) >>> } >>> >>> struct page_frag_cache { >>> - void * va; >>> -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) >>> - __u16 offset; >>> - __u16 size; >>> -#else >>> - __u32 offset; >>> -#endif >>> + struct folio *folio; >>> + unsigned int offset; >>> /* we maintain a pagecount bias, so that we dont dirty cache line >>> * containing page->_refcount every time we allocate a fragment. >>> */ >>> - unsigned int pagecnt_bias; >>> - bool pfmemalloc; >>> + unsigned int pagecnt_bias; >>> + bool pfmemalloc; >>> }; >> >> It seems 'va' and 'size' field is used to avoid touching 'stuct page' to >> avoid possible cache bouncing when there is more frag can be allocated >> from the page while other frags is freed at the same time before this patch? > > Hmmm... fair point, though va is calculated from the page pointer on most > arches without the need to dereference struct page (only arc, m68k and sparc > define WANT_PAGE_VIRTUAL). > > David > --Mika
On Wed, 24 May 2023 16:33:03 +0100 David Howells wrote: > - offset = nc->offset - fragsz; > - if (unlikely(offset < 0)) { > - page = virt_to_page(nc->va); > - > - if (page_ref_count(page) != nc->pagecnt_bias) > + offset = nc->offset; > + if (unlikely(fragsz > offset)) { > + /* Reuse the folio if everyone we gave it to has finished with > + * it. > + */ > + if (!folio_ref_sub_and_test(folio, nc->pagecnt_bias)) { > + nc->folio = NULL; > goto refill; > + } > + > if (unlikely(nc->pfmemalloc)) { > - page_ref_sub(page, nc->pagecnt_bias - 1); > - __free_pages(page, compound_order(page)); > + __folio_put(folio); This is not a pure 1:1 page -> folio conversion. Why mix conversion with other code changes?
On Fri, 2023-05-26 at 19:56 +0800, Yunsheng Lin wrote: > On 2023/5/24 23:33, David Howells wrote: > > Change the page_frag_cache allocator to use multipage folios rather than > > groups of pages. This reduces page_frag_free to just a folio_put() or > > put_page(). > > Hi, David > > put_page() is not used in this patch, perhaps remove it to avoid > the confusion? > Also, Is there any significant difference between __free_pages() > and folio_put()? IOW, what does the 'reduces' part means here? > > I followed some disscusion about folio before, but have not really > understood about real difference between 'multipage folios' and > 'groups of pages' yet. Is folio mostly used to avoid the confusion > about whether a page is 'headpage of compound page', 'base page' or > 'tailpage of compound page'? Or is there any abvious benefit about > folio that I missed? > > > > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > > index 306a3d1a0fa6..d7c52a5979cc 100644 > > --- a/include/linux/mm_types.h > > +++ b/include/linux/mm_types.h > > @@ -420,18 +420,13 @@ static inline void *folio_get_private(struct folio *folio) > > } > > > > struct page_frag_cache { > > - void * va; > > -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) > > - __u16 offset; > > - __u16 size; > > -#else > > - __u32 offset; > > -#endif > > + struct folio *folio; > > + unsigned int offset; > > /* we maintain a pagecount bias, so that we dont dirty cache line > > * containing page->_refcount every time we allocate a fragment. > > */ > > - unsigned int pagecnt_bias; > > - bool pfmemalloc; > > + unsigned int pagecnt_bias; > > + bool pfmemalloc; > > }; > > It seems 'va' and 'size' field is used to avoid touching 'stuct page' to > avoid possible cache bouncing when there is more frag can be allocated > from the page while other frags is freed at the same time before this patch? > It might be worth calling that out in the commit log or split it into another > patch to make it clearer and easier to review? Yes, there is a cost for going from page to virtual address. That is why we only use the page when we finally get to freeing or resetting the pagecnt_bias. Also I have some concerns about going from page to folio as it seems like the folio_alloc setups the transparent hugepage destructor instead of using the compound page destructor. I would think that would slow down most users as it looks like there is a spinlock that is taken in the hugepage destructor that isn't there in the compound page destructor.
Alexander H Duyck <alexander.duyck@gmail.com> wrote: > Also I have some concerns about going from page to folio as it seems > like the folio_alloc setups the transparent hugepage destructor instead > of using the compound page destructor. I would think that would slow > down most users as it looks like there is a spinlock that is taken in > the hugepage destructor that isn't there in the compound page > destructor. Note that this code is going to have to move to folios[*] at some point. "Old-style" compound pages are going to go away, I believe. Matthew Wilcox and the mm folks are on a drive towards simplifying memory management, formalising chunks larger than a single page - with the ultimate aim of reducing the page struct to a single, typed pointer. So, take, for example, a folio: As I understand it, this will no longer overlay struct page, but rather will become a single, dynamically-allocated struct that covers a pow-of-2 number of pages. A contiguous subset of page structs will point at it. However, rather than using a folio, we could define a "page fragment" memory type. Rather than having all the flags and fields to be found in struct folio, it could have just the set to be found in page_frag_cache. David [*] It will be possible to have some other type than "folio". See "struct slab" in mm/slab.h for example. struct slab corresponds to a set of pages and, in the future, a number of struct pages will point at it.
On Tue, Jun 6, 2023 at 1:25 AM David Howells <dhowells@redhat.com> wrote: > > Alexander H Duyck <alexander.duyck@gmail.com> wrote: > > > Also I have some concerns about going from page to folio as it seems > > like the folio_alloc setups the transparent hugepage destructor instead > > of using the compound page destructor. I would think that would slow > > down most users as it looks like there is a spinlock that is taken in > > the hugepage destructor that isn't there in the compound page > > destructor. > > Note that this code is going to have to move to folios[*] at some point. > "Old-style" compound pages are going to go away, I believe. Matthew Wilcox > and the mm folks are on a drive towards simplifying memory management, > formalising chunks larger than a single page - with the ultimate aim of > reducing the page struct to a single, typed pointer. I'm not against making the move, but as others have pointed out this is getting into unrelated things. One of those being the fact that to transition to using folios we don't need to get rid of the use of the virtual address. The idea behind using the virtual address here is that we can avoid a bunch of address translation overhead since we only need to use the folio if we are going to allocate, retire, or recycle a page/folio. If we are using an order 3 page that shouldn't be very often. > So, take, for example, a folio: As I understand it, this will no longer > overlay struct page, but rather will become a single, dynamically-allocated > struct that covers a pow-of-2 number of pages. A contiguous subset of page > structs will point at it. > > However, rather than using a folio, we could define a "page fragment" memory > type. Rather than having all the flags and fields to be found in struct > folio, it could have just the set to be found in page_frag_cache. I don't think we need a new memory type. For the most part the page fragment code is really more a subset of something like a __get_free_pages where the requester provides the size, is just given a virtual address, and we shouldn't need to be allocating a new page as often as ideally the allocations are 2K or less in size. Also one thing I would want to avoid is adding complexity to the freeing path. The general idea with page frags is that they are meant to be lightweight in terms of freeing as well. So just as they are similar to __get_free_pages in terms of allocation the freeing is meant to be similar to free_pages. > David > > [*] It will be possible to have some other type than "folio". See "struct > slab" in mm/slab.h for example. struct slab corresponds to a set of pages > and, in the future, a number of struct pages will point at it. I want to avoid getting anywhere near the complexity of a slab allocator. The whole point of this was to keep it simple so that drivers could use it and get decent performance. When I had implemented it in the Intel drivers back in the day this approach was essentially just a reference count/page offset hack that allowed us to split a page in 2 and use the pages as a sort of mobius strip within the ring buffer.
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 306a3d1a0fa6..d7c52a5979cc 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -420,18 +420,13 @@ static inline void *folio_get_private(struct folio *folio) } struct page_frag_cache { - void * va; -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) - __u16 offset; - __u16 size; -#else - __u32 offset; -#endif + struct folio *folio; + unsigned int offset; /* we maintain a pagecount bias, so that we dont dirty cache line * containing page->_refcount every time we allocate a fragment. */ - unsigned int pagecnt_bias; - bool pfmemalloc; + unsigned int pagecnt_bias; + bool pfmemalloc; }; typedef unsigned long vm_flags_t; diff --git a/mm/page_frag_alloc.c b/mm/page_frag_alloc.c index 9d3f6fbd9a07..ffd68bfb677d 100644 --- a/mm/page_frag_alloc.c +++ b/mm/page_frag_alloc.c @@ -16,33 +16,34 @@ #include <linux/init.h> #include <linux/mm.h> -static struct page *__page_frag_cache_refill(struct page_frag_cache *nc, - gfp_t gfp_mask) +/* + * Allocate a new folio for the frag cache. + */ +static struct folio *page_frag_cache_refill(struct page_frag_cache *nc, + gfp_t gfp_mask) { - struct page *page = NULL; + struct folio *folio = NULL; gfp_t gfp = gfp_mask; #if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) - gfp_mask |= __GFP_COMP | __GFP_NOWARN | __GFP_NORETRY | - __GFP_NOMEMALLOC; - page = alloc_pages_node(NUMA_NO_NODE, gfp_mask, - PAGE_FRAG_CACHE_MAX_ORDER); - nc->size = page ? PAGE_FRAG_CACHE_MAX_SIZE : PAGE_SIZE; + gfp_mask |= __GFP_NOWARN | __GFP_NORETRY | __GFP_NOMEMALLOC; + folio = folio_alloc(gfp_mask, PAGE_FRAG_CACHE_MAX_ORDER); #endif - if (unlikely(!page)) - page = alloc_pages_node(NUMA_NO_NODE, gfp, 0); + if (unlikely(!folio)) + folio = folio_alloc(gfp, 0); - nc->va = page ? page_address(page) : NULL; - - return page; + if (folio) + nc->folio = folio; + return folio; } void __page_frag_cache_drain(struct page *page, unsigned int count) { - VM_BUG_ON_PAGE(page_ref_count(page) == 0, page); + struct folio *folio = page_folio(page); + + VM_BUG_ON_FOLIO(folio_ref_count(folio) == 0, folio); - if (page_ref_sub_and_test(page, count - 1)) - __free_pages(page, compound_order(page)); + folio_put_refs(folio, count); } EXPORT_SYMBOL(__page_frag_cache_drain); @@ -54,11 +55,12 @@ EXPORT_SYMBOL(__page_frag_cache_drain); */ void page_frag_cache_clear(struct page_frag_cache *nc) { - if (nc->va) { - struct page *page = virt_to_head_page(nc->va); + struct folio *folio = nc->folio; - __page_frag_cache_drain(page, nc->pagecnt_bias); - nc->va = NULL; + if (folio) { + VM_BUG_ON_FOLIO(folio_ref_count(folio) == 0, folio); + folio_put_refs(folio, nc->pagecnt_bias); + nc->folio = NULL; } } EXPORT_SYMBOL(page_frag_cache_clear); @@ -67,56 +69,51 @@ void *page_frag_alloc_align(struct page_frag_cache *nc, unsigned int fragsz, gfp_t gfp_mask, unsigned int align) { - unsigned int size = PAGE_SIZE; - struct page *page; - int offset; + struct folio *folio = nc->folio; + size_t offset; WARN_ON_ONCE(!is_power_of_2(align)); - if (unlikely(!nc->va)) { + if (unlikely(!folio)) { refill: - page = __page_frag_cache_refill(nc, gfp_mask); - if (!page) + folio = page_frag_cache_refill(nc, gfp_mask); + if (!folio) return NULL; -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) - /* if size can vary use size else just use PAGE_SIZE */ - size = nc->size; -#endif /* Even if we own the page, we do not use atomic_set(). * This would break get_page_unless_zero() users. */ - page_ref_add(page, PAGE_FRAG_CACHE_MAX_SIZE); + folio_ref_add(folio, PAGE_FRAG_CACHE_MAX_SIZE); /* reset page count bias and offset to start of new frag */ - nc->pfmemalloc = page_is_pfmemalloc(page); + nc->pfmemalloc = folio_is_pfmemalloc(folio); nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1; - nc->offset = size; + nc->offset = folio_size(folio); } - offset = nc->offset - fragsz; - if (unlikely(offset < 0)) { - page = virt_to_page(nc->va); - - if (page_ref_count(page) != nc->pagecnt_bias) + offset = nc->offset; + if (unlikely(fragsz > offset)) { + /* Reuse the folio if everyone we gave it to has finished with + * it. + */ + if (!folio_ref_sub_and_test(folio, nc->pagecnt_bias)) { + nc->folio = NULL; goto refill; + } + if (unlikely(nc->pfmemalloc)) { - page_ref_sub(page, nc->pagecnt_bias - 1); - __free_pages(page, compound_order(page)); + __folio_put(folio); + nc->folio = NULL; goto refill; } -#if (PAGE_SIZE < PAGE_FRAG_CACHE_MAX_SIZE) - /* if size can vary use size else just use PAGE_SIZE */ - size = nc->size; -#endif /* OK, page count is 0, we can safely set it */ - set_page_count(page, PAGE_FRAG_CACHE_MAX_SIZE + 1); + folio_set_count(folio, PAGE_FRAG_CACHE_MAX_SIZE + 1); /* reset page count bias and offset to start of new frag */ nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1; - offset = size - fragsz; - if (unlikely(offset < 0)) { + offset = folio_size(folio); + if (unlikely(fragsz > offset)) { /* * The caller is trying to allocate a fragment * with fragsz > PAGE_SIZE but the cache isn't big @@ -126,15 +123,17 @@ void *page_frag_alloc_align(struct page_frag_cache *nc, * it could make memory pressure worse * so we simply return NULL here. */ + nc->offset = offset; return NULL; } } nc->pagecnt_bias--; + offset -= fragsz; offset &= ~(align - 1); nc->offset = offset; - return nc->va + offset; + return folio_address(folio) + offset; } EXPORT_SYMBOL(page_frag_alloc_align); @@ -143,8 +142,6 @@ EXPORT_SYMBOL(page_frag_alloc_align); */ void page_frag_free(void *addr) { - struct page *page = virt_to_head_page(addr); - - __free_pages(page, compound_order(page)); + folio_put(virt_to_folio(addr)); } EXPORT_SYMBOL(page_frag_free);
Change the page_frag_cache allocator to use multipage folios rather than groups of pages. This reduces page_frag_free to just a folio_put() or put_page(). Signed-off-by: David Howells <dhowells@redhat.com> cc: "David S. Miller" <davem@davemloft.net> cc: Eric Dumazet <edumazet@google.com> cc: Jakub Kicinski <kuba@kernel.org> cc: Paolo Abeni <pabeni@redhat.com> cc: Jens Axboe <axboe@kernel.dk> cc: Jeroen de Borst <jeroendb@google.com> cc: Catherine Sullivan <csully@google.com> cc: Shailend Chand <shailend@google.com> cc: Felix Fietkau <nbd@nbd.name> cc: John Crispin <john@phrozen.org> cc: Sean Wang <sean.wang@mediatek.com> cc: Mark Lee <Mark-MC.Lee@mediatek.com> cc: Lorenzo Bianconi <lorenzo@kernel.org> cc: Matthias Brugger <matthias.bgg@gmail.com> cc: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com> cc: Keith Busch <kbusch@kernel.org> cc: Jens Axboe <axboe@fb.com> cc: Christoph Hellwig <hch@lst.de> cc: Sagi Grimberg <sagi@grimberg.me> cc: Chaitanya Kulkarni <kch@nvidia.com> cc: Andrew Morton <akpm@linux-foundation.org> cc: Matthew Wilcox <willy@infradead.org> cc: netdev@vger.kernel.org cc: linux-arm-kernel@lists.infradead.org cc: linux-mediatek@lists.infradead.org cc: linux-nvme@lists.infradead.org cc: linux-mm@kvack.org --- include/linux/mm_types.h | 13 ++---- mm/page_frag_alloc.c | 99 +++++++++++++++++++--------------------- 2 files changed, 52 insertions(+), 60 deletions(-)