Messages in this thread | | | Date | Thu, 05 Jul 2007 12:04:23 +1000 | From | Nick Piggin <> | Subject | Re: [BUGFIX][PATCH] DO flush icache before set_pte() on ia64. |
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KAMEZAWA Hiroyuki wrote: > On Wed, 04 Jul 2007 16:31:06 +1000 > Nick Piggin <nickpiggin@yahoo.com.au> wrote: > >>The only thing I noticed when I looked at the code is that some places >>may not have flushed icache when they should have? Did you get them all? > > > I think that I added flush_icache_page() to the place where any flush_(i)cache_xxx > is not called and lazy_mmu_prot_update was used instead of them. > But I want good review, of course. > > >>Minor nitpick: you have one place where you test VM_EXEC before flushing, >>but the flush routine itself contains the same test I think? >> > > Ah, yes...in do_anonymous_page(). my mistake. > > >>Regarding the ia64 code -- I'm not an expert so I can't say whether it >>is the right thing to do or not. However I still can't work out what it's >>rationale for the PG_arch_1 bit is, exactly. Does it assume that >>flush_dcache_page sites would only ever be encountered by pages that are >>not faulted in? A faulted in page kind of is "special" because it is >>guaranteed uptodate, but is the ia64 arch code relying on that? Should it? > > > (I'm sorry if I misses point.) > ia64's D-cache is coherent but I-cache and D-cache is not coherent and any > invalidation against d-cache will invalidate I-cache. > > In my understanding : > PG_arch_1 is used for showing "there is no inconsistent data on any level of > cache". PG_uptodate is used for showing "this page includes the newest data > and contents are valid." > ...maybe not used for the same purpose.
I think that's right, but why is set_pte-time the critical point for the flush? It is actually possible to write into an executable page via the dcache *after* it has ptes pointing to it.
From what I can work out, it is something like "at this point the page should be uptodate, so at least the icache won't contain *inconsistent* data, just old data which userspace should take care of flushing if it modifies". Is that always true? Could the page get modified by means other than a direct write(2)? And even in the case of a write(2) writer, how do they know if another process is mapping that particular page for exec at that time? Should they always flush? Flushing would require they have a virtual address on the page to begin with anyway, doesn't it? So they'd have to mmap it... phew.
I guess it is mostly safe because it is probably very uncommon to do such a thing, and chances are no non-write(2) write activity happens to a page after it is brought uptodate. But I don't know if that has been audited. I would really like to see the kernel always manage all aspects of its pagecache though. I realise performance considerations may make this not always possible... but it might be possible to do efficiently using mapcount these days?
Anyway, ignore my tangent if you like :) Your patch doesn't make any of this worse, so I'm getting off topic.
So I think your patch is nice, but would need ia64 people to actually ack it.
> BTW, a page filled by DMA should have PG_arch_1 :(
The consequences of not are superfluous flushes?
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