Re: [patch] mm: mremap correct rmap accounting

[Carsten Otte <cotte@de.ibm.com>]

Hugh Dickins wrote:
> I think it's now clear that XIP won't be impacted at all by my
> ZERO_PAGE(0) change, and that's the patch Linus should put in
> for 2.6.20 (given how much he disliked Nick's patch to maintain
> the different zeropage counts across mremap move).  Ah, good,
> that's now gone into his tree since last I looked.
Good to know this issue is solved. Thank you.

> But there is a change which I now think you do need to make,
> for 2.6.21 - let it not distract attention from the pagecount
> correctness issue we've been discussing so far.  Something I
> should have noticed when I first looked at your clever use of
> the ZERO_PAGE, but have only noticed now.  Doesn't it clash
> with the clever use of the ZERO_PAGE when reading /dev/zero
> (see read_zero_pagealigned in drivers/char/mem.c)?
> 
> Consider two PROT_READ|PROT_WRITE,MAP_PRIVATE mappings of a
> four-page hole in a XIP file.  One of them just readfaults the
> four pages in (and is given ZERO_PAGE for each), the other has
> four pages read from /dev/zero into it (which also maps the
> ZERO_PAGE into its four ptes).
> 
> Then imagine that non-zero data is written to the first page of
> that hole, by a write syscall, or through a PROT_WRITE,MAP_SHARED
> mapping.  __xip_unmap will replace the first ZERO_PAGE in each of
> the MAP_PRIVATE mappings by the new non-zero data page.  Which is
> correct for the first mapping which just did readfaults, but wrong
> for the second mapping which has overwritten by reading /dev/zero
> - those pages ought to remain zeroed, never seeing the later data.
Wait a second, I fail to see why those pages in the second mapping 
should remain zeroed and never see the later data; they do reflect the 
file content that was mmap()ed there correctly at all times. I would 
expect this behavior, and would expect to see both mappings reflect 
this data. Am I missing something?

> Or have I got it wrong?  A simple test should show.
The test is simple indeed, will do that - and compare page cache 
backed file behavior with xip file.

Carsten

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