Hardy: Reinstate ZERO_PAGE optimization in 'get_user_pages()' and fix XIP

[Tim Gardner]

Upstream cherry-pick requested by Kees Cook.

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 ]

[Tim Gardner (timg-tpi)]

http://kernel.ubuntu.com/git?p=ubuntu/ubuntu-hardy.git;a=commit;h=d6292ae514cbb772805c5b6492800f4e38272e52

[Tim Gardner (timg-tpi)]

SRU Justification: see bug description

[Steve Langasek (vorlon)]

Accepted into -proposed, please test and give feedback here. Please see https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you in advance!

[Michael Rooney (mrooney)]

I'll be happy to try it out, but I don't know what I should test for specifically, if anything. Let me know if I can help!

[Martin Pitt (pitti)]

linux 2.6.24-21 copied to hardy-updates.