[RFC][PATCH 3/4] get_user_pages READ fault handling special cases
From: KAMEZAWA Hiroyuki <hidden>
Date: 2009-07-07 07:18:44
most of parts are overwritten by 4/4 patch. == From: KAMEZAWA Hiroyuki <redacted> Now, get_user_pages(READ) can return ZERO_PAGE but it creates some trouble. This patch is a workaround for each callers. - mlock() ....ignore ZERO_PAGE if found. This happens only when mlock against read-only mapping finds zero pages. - futex() ....if ZERO PAGE is found....BUG ?(but possible...) - lookup_node() .... no good idea..this is the same behavior to 2.6.23 age. Others ? I wonder it's better to add some function to replace ZERO PAGE to be an usual page..(do copy-on-write if ZERO_PAGE) in some special cases. (like futex in above.) Signed-off-by: KAMEZAWA Hiroyuki <redacted> --- Index: zeropage-trial/mm/mlock.c ===================================================================
--- zeropage-trial.orig/mm/mlock.c
+++ zeropage-trial/mm/mlock.c@@ -220,19 +220,24 @@ static long __mlock_vma_pages_range(stru for (i = 0; i < ret; i++) { struct page *page = pages[i]; - lock_page(page); + /* * Because we lock page here and migration is blocked * by the elevated reference, we need only check for * page truncation (file-cache only). + * + * The page can be ZERO_PAGE if VM_WRITE is not set. */ - if (page->mapping) { - if (mlock) - mlock_vma_page(page); - else - munlock_vma_page(page); + if (!page_is_zero(page)) { + lock_page(page); + if (page->mapping) { + if (mlock) + mlock_vma_page(page); + else + munlock_vma_page(page); + } + unlock_page(page); } - unlock_page(page); put_page(page); /* ref from get_user_pages() */ /*
Index: zeropage-trial/kernel/futex.c ===================================================================
--- zeropage-trial.orig/kernel/futex.c
+++ zeropage-trial/kernel/futex.c@@ -249,9 +249,23 @@ again: lock_page(page); if (!page->mapping) { + if (!page_is_zero(page)) { + unlock_page(page); + put_page(page); + goto again; + } + /* + * Finding ZERO PAGE here is obviously user's BUG because + * futex_wake()etc. is called against never-written page. + * Considering how futex is used, this kind of bug should not + * happen i.e. very strange system bug. Then, print out message. + */ unlock_page(page); put_page(page); - goto again; + printk(KERN_WARNING "futex is called against not-initialized" + "memory %d(%s) at %p", current->pid, + current->comm, (void*)address); + return -EINVAL; } /*
Index: zeropage-trial/mm/mempolicy.c ===================================================================
--- zeropage-trial.orig/mm/mempolicy.c
+++ zeropage-trial/mm/mempolicy.c@@ -684,6 +684,10 @@ static int lookup_node(struct mm_struct struct page *p; int err; + /* + * This get_user_page() may catch ZERO PAGE. In that case, returned + * value will not be very useful. But we can't return error here. + */ err = get_user_pages(current, mm, addr & PAGE_MASK, 1, 0, 0, &p, NULL); if (err >= 0) { err = page_to_nid(p); --
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