On Thu, 05 Jul 2007 12:04:23 +1000
Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
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.
yes. I think there are 2 cases.
- copy-on-write case..... OS handles this.
- page is writable and app just rewrites it ...... the app should handle this.
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?
I think it's true. But, in this case, i-cache doesn't contain *incositent* data.
There are inconsistency between L2-Dcache and L3-mixed-cache. At L2-icache-miss,
a cpu fetches data from L3 cache.
This case seems defficult to be generalized...
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?
generic_file_write() does flush_dcahe_page() but no flush_icache_page()...
Then..maybe this will be necessary...
==
flush_dcache_page(page);
if (page_mapcount(page) > 0 && page_is_mapped_as_text(page))
flush_icache_page(page);
==
But I don't know whether write(2) to mapped text file is expected to work well.