On Mon, Jan 25, 2016 at 9:33 AM, Andy Lutomirski [off-list ref] wrote:

On Mon, Jan 25, 2016 at 9:25 AM, Matthew Wilcox
[off-list ref] wrote:

quoted

From: Matthew Wilcox <redacted>

track_pfn_insert() overwrites the pgprot that is passed in with a value
based on the VMA's page_prot.  This is a problem for people trying to
do clever things with the new vm_insert_pfn_prot() as it will simply
overwrite the passed protection flags.  If we use the current value of
the pgprot as the base, then it will behave as people are expecting.

Also fix track_pfn_remap() in the same way.

Well that's embarrassing.  Presumably it worked for me because I only
overrode the cacheability bits and lookup_memtype did the right thing.

But shouldn't the PAT code change the memtype if vm_insert_pfn_prot
requests it?  Or are there no callers that actually need that?  (HPET
doesn't, because there's a plain old ioremapped mapping.)

Looking a bit further, track_pfn_remap does this, while
track_pfn_insert does not.  I don't know why

I'm also a bit confused as to how any of this works.  There doesn't
seem to be any reference counting of memtypes, so I don't understand
why, say, remapping the same range twice and then freeing them in FIFO
order doesn't break the memtype code.  (There's VM_PAT, but that seems
likely to be extremely fragile.)

--Andy

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>