From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Wed, 7 May 2008 16:36:43 +0900 From: KAMEZAWA Hiroyuki Subject: [PATCH] more ZERO_PAGE handling in follow_page() Message-Id: <20080507163643.d4da0ed0.kamezawa.hiroyu@jp.fujitsu.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: LKML Cc: "linux-mm@kvack.org" , Andrew Morton , "nickpiggin@yahoo.com.au" , tonyb@cybernetics.com, mika.penttila@kolumbus.fi List-ID: Rewrote the description of patch. (no changes in the logic.) Thank you for all help. -Kame == follow_page() is called from get_user_pages(), which returns specified user page. follow_page() can return 1) a page or 2) NULL or 3)ZERO_PAGE. If NULL, handle_mm_fault() is called. Now, follow_page() to unused pte returns NULL if page table exists. As a result get_user_pages() calls handle_mm_fault() and allocate new memory. This behavior increases memory consumption at coredump, which does read-once-but-never-written page fault. By returning ZERO_PAGE() against READ/ANON request, we can avoid it. (Because exec's arguments copy needs to call handle_mm_fault at WRITE/ANON request, we just handle READ/ANON case here.) Change log: - Rewrote patch description and Added comments. - fixed to check pte_present()/pte_none() in proper way. Signed-off-by: KAMEZAWA Hiroyuki Tested-by: Tony Battersby Acked-by: Nick Piggin --- mm/memory.c | 16 +++++++++++----- 1 file changed, 11 insertions(+), 5 deletions(-) Index: linux-2.6.25/mm/memory.c =================================================================== --- linux-2.6.25.orig/mm/memory.c +++ linux-2.6.25/mm/memory.c @@ -926,15 +926,15 @@ struct page *follow_page(struct vm_area_ page = NULL; pgd = pgd_offset(mm, address); if (pgd_none(*pgd) || unlikely(pgd_bad(*pgd))) - goto no_page_table; + goto null_or_zeropage; pud = pud_offset(pgd, address); if (pud_none(*pud) || unlikely(pud_bad(*pud))) - goto no_page_table; + goto null_or_zeropage; pmd = pmd_offset(pud, address); if (pmd_none(*pmd) || unlikely(pmd_bad(*pmd))) - goto no_page_table; + goto null_or_zeropage; if (pmd_huge(*pmd)) { BUG_ON(flags & FOLL_GET); @@ -947,8 +947,14 @@ struct page *follow_page(struct vm_area_ goto out; pte = *ptep; - if (!pte_present(pte)) + if (!pte_present(pte)) { + /* Read fault to empty pte can return ZERO_PAGE */ + if (!(flags & FOLL_WRITE) && pte_none(pte)) { + pte_unmap_unlock(ptep, ptl); + goto null_or_zeropage; + } goto unlock; + } if ((flags & FOLL_WRITE) && !pte_write(pte)) goto unlock; page = vm_normal_page(vma, address, pte); @@ -968,7 +974,7 @@ unlock: out: return page; -no_page_table: +null_or_zeropage: /* * When core dumping an enormous anonymous area that nobody * has touched so far, we don't want to allocate page tables. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org