Re: [PATCH net-next 04/12] mm: Make the page_frag_cache allocator use multipage folios
From: Yunsheng Lin <hidden>
Date: 2023-05-26 11:56:22
Also in:
linux-arm-kernel, linux-mediatek, linux-mm, linux-nvme, lkml
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?
quoted hunk ↗ jump to hunk
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?