Re: [PATCH] ARM/shmem: Drop page coloring align for non-VIPT CPUs

From: Russell King - ARM Linux
Date: Tue May 23 2017 - 16:10:50 EST

On Thu, May 18, 2017 at 02:22:57PM +0300, Dmitry Safonov wrote:
> On 04/25/2017 08:35 PM, Russell King - ARM Linux wrote:
> >On Tue, Apr 25, 2017 at 08:19:21PM +0300, Dmitry Safonov wrote:
> >>On 04/14/2017 01:09 PM, Dmitry Safonov wrote:
> >>>On ARMv6 CPUs with VIPT caches there are aliasing issues: if two
> >>>different cache line indexes correspond to the same physical
> >>>address, then changes made to one of the alias might be lost
> >>>or they can overwrite each other. To overcome aliasing issues,
> >>>the align for shared mappings was introduced with:
> >>>
> >>>commit 4197692eef113eeb8e3e413cc70993a5e667e5b8
> >>>Author: Russell King <rmk@xxxxxxxxxxxxxxxxxxxxxx>
> >>>Date: Wed Apr 28 22:22:33 2004 +0100
> >>>
> >>> [ARM] Fix shared mmap()ings for ARM VIPT caches.
> >>>
> >>> This allows us to appropriately align shared mappings on VIPT caches
> >>> with aliasing issues.
> >>>
> >>>Which introduced 4 pages align with SHMLBA, which resulted in
> >>>unique physical address after any tag in cache (because two upper bits
> >>>corresponding to page address get unused in tags).
> >>>
> >>>As this workaround is not needed by non-VIPT caches (like most armv7
> >>>CPUs which have PIPT caches), ARM mmap() code checks if cache is VIPT
> >>>aliasing for MAP_SHARED.
> >>>
> >>>The problem here is in shmat() syscall:
> >>>1. if shmaddr is NULL then do_shmat() uses arch_get_unmapped_area()
> >>> to allocate shared mapping.
> >>>2. if shmaddr is specified then do_shmat() checks that address has
> >>> SHMLBA alignment regardless to CPU cache aliasing.
> >>>
> >>>Which results on ARMv7 CPUs that shmat() with NULL shmaddr may return
> >>>non-SHMLBA aligned address (page-aligned), but shmat() with the same
> >>>address will fail.
> >>>
> >>>That is not critical issue for CRIU as after shmat() with NULL address,
> >
> >CRIU? Please try to keep use of acronyms to a minimum.
> >
> >>>we can mremap() resulted shmem to restore shared memory mappings on the
> >>>same address where they were on checkpointing.
> >>>But it's still worth fixing because we can't reliably tell from
> >>>userspace if the platform has VIPT cache, and so this mremap()
> >>>workaround is done with HUGE warning that restoring application, that
> >>>uses SHMBLA-unaligned shmem on ARMv6 CPU with VIPT cache may result
> >>>in data corruptions.
> >>>
> >>>I also changed SHMLBA build-time check to init-time WARN_ON(), as
> >>>it's not constant afterward.
> >
> >I'm not happy with this. SHMLBA is defined by POSIX to be a constant,
> >which means that if we want to have any kind of binary compatibility
> >between different architecture versions, SHMLBA must be constant across
> >all variants of the architecture.
> >
> >Making it dependent on the cache architecture means that userspace's
> >assumptions can be broken. Increasing it is not an issue (since SHMLBA
> >is defined to be the address multiple - an address that is aligned to
> >4-page is also by definition aligned to 1-page.) So what I did back in
> >2004 wasn't a problem.
> >
> >However, reducing it (as you're now suggesting) is - newly built programs
> >are built today with:
> >
> >#define SHMLBA (__getpagesize () << 2)
> >
> >so we must not allow the kernel to return addresses that violate that.
> >As I say, we can't reduce SHMLBA now.
> So, we violate this on return address with shmat(smid, NULL, shmflg)
> when shmaddr == 0.
> But we don't do this on shmat(smid, shmaddr, shmflg)
> where shmaddr should be SHMLBA-aligned.
> That API looks unexpected and creates difficulties, which I've
> workarounded in CRIU, but still might worth fixing.

It should at least be self-consistent.

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