From: Linus Torvalds Date: Fri, 20 Jun 2008 18:18:25 +0000 (-0700) Subject: Reinstate ZERO_PAGE optimization in 'get_user_pages()' and fix XIP X-Git-Tag: v2.6.26-rc7~12 X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=89f5b7da2a6bad2e84670422ab8192382a5aeb9f;p=pandora-kernel.git Reinstate ZERO_PAGE optimization in 'get_user_pages()' and fix XIP KAMEZAWA Hiroyuki and Oleg Nesterov point out that since the commit 557ed1fa2620dc119adb86b34c614e152a629a80 ("remove ZERO_PAGE") removed the ZERO_PAGE from the VM mappings, any users of get_user_pages() will generally now populate the VM with real empty pages needlessly. We used to get the ZERO_PAGE when we did the "handle_mm_fault()", but since fault handling no longer uses ZERO_PAGE for new anonymous pages, we now need to handle that special case in follow_page() instead. In particular, the removal of ZERO_PAGE effectively removed the core file writing optimization where we would skip writing pages that had not been populated at all, and increased memory pressure a lot by allocating all those useless newly zeroed pages. This reinstates the optimization by making the unmapped PTE case the same as for a non-existent page table, which already did this correctly. While at it, this also fixes the XIP case for follow_page(), where the caller could not differentiate between the case of a page that simply could not be used (because it had no "struct page" associated with it) and a page that just wasn't mapped. We do that by simply returning an error pointer for pages that could not be turned into a "struct page *". The error is arbitrarily picked to be EFAULT, since that was what get_user_pages() already used for the equivalent IO-mapped page case. [ Also removed an impossible test for pte_offset_map_lock() failing: that's not how that function works ] Acked-by: Oleg Nesterov Acked-by: Nick Piggin Cc: KAMEZAWA Hiroyuki Cc: Hugh Dickins Cc: Andrew Morton Cc: Ingo Molnar Cc: Roland McGrath Signed-off-by: Linus Torvalds --- Reading git-diff-tree failed