On Thu, 2008-11-20 at 09:19 +0000, Russell King - ARM Linux wrote:
> On Thu, Nov 20, 2008 at 05:59:00PM +1100, Nick Piggin wrote:
> > - The page is sent to the block layer, which stores into the page. Some
> > block devices like 'brd' will potentially store via the kernel linear map
> > here, and they probably don't do enough cache flushing. But a regular
> > block device should go via DMA, which AFAIK should be OK? (the user address
> > should remain invalidated because it would be a bug to read from the buffer
> > before the read has completed)
>
> This is where things get icky with lots of drivers - DMA is fine, but
> many PIO based drivers don't handle the implications of writing to the
> kernel page cache page when there may be CPU cache side effects.
And for PIO devices, what cache flushing function should be used if the
page isn't available (maybe just a pointer to a buffer) to do a
flush_dcache_page? Should this be done in the filesystem layer?
> If the cache is in read allocate mode, then in this case there shouldn't
> be any dirty cache lines. (That's not always the case though, esp. via
> conventional IO.) If the cache is in write allocate mode, PIO data will
> sit in the kernel mapping and won't be visible to userspace.
This problem re-appeared in our tests when someone started using an ext2
filesystem with mtd+slram with write-allocate caches. Basically the D
cache isn't flushed to ensure its coherency with the I cache.
The very dirty workaround was to use flush_icache_range (I know, it's
meant for something completely different) in mtd_blkdevs.c. The slram.c
doesn't have access to any page information either and I noticed that
there are very few block devices that call flush_dcache_page directly.
Should this be done by the filesystem layer?
It looks to me like some filesystems like cramfs call flush_dcache_page
in their "readpage" functions but ext2 doesn't. My other, less dirty,
workaround for the mtd+slram problem is below. It appears to solve the
problem though I'm not sure it is the right solution (ext2 uses
mpage_readpages).
diff --git a/fs/mpage.c b/fs/mpage.c
index 552b80b..979a4a9 100644
--- a/fs/mpage.c
+++ b/fs/mpage.c
@@ -51,6 +51,7 @@ static void mpage_end_io_read(struct bio *bio, int err)
prefetchw(&bvec->bv_page->flags);
if (uptodate) {
+ flush_dcache_page(page);
SetPageUptodate(page);
} else {
ClearPageUptodate(page);
Thanks.
--
Catalin
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