Re: How to use huge pages in drivers?

[Dominique Martinet <asmadeus@codewreck.org>]

Dominique Martinet wrote on Tue, Sep 03, 2019:
> Matthew Wilcox wrote on Tue, Sep 03, 2019:
> > > What I'd like to know is:
> > >  - we know (assuming the other side isn't too bugged, but if it is we're
> > > fucked up anyway) exactly what huge-page-sized physical memory range has
> > > been mapped on the other side, is there a way to manually gather the
> > > pages corresponding and merge them into a huge page?
> > 
> > You're using the word 'page' here, but I suspect what you really mean is
> > "pfn" or "pte".  As you've described it, it doesn't matter what data structure
> > Linux is using for the memory, since Linux doesn't know about the memory.
> 
> Correct, we're already using vmf_insert_pfn

Actually let me take that back, vmf_insert_pfn is only used if
pfn_valid() is false, probably as a safeguard of sort(?).
The normal case went with pfn_to_page(pfn) + vm_insert_page() so, as
things stands.
I do have a few more questions if you could humor me a bit more...

 - the vma was created with a vm_flags including VM_MIXEDMAP for some
reason, I don't know why.
If I change it to VM_PFNMAP (which sounds better here from the little I
understand of this as we do not need cow and looks a bit simpler?), I
can remove the vm_insert_page() path and use the vmf_insert_pfn one
instead, which appears to work fine for simple programs... But the
kernel thread for my network adapter (bxi... which is not upstream
either I guess.. sigh..) no longer tries to fault via my custom .fault
vm operation... Which means I probably did need MIXEDMAP ?

I'm honestly not sure where to read up on what these two flags imply,
looking at the page fault handler code I do not see why the request from
a kernel thread would care what kind of vma it is...


 - ignoring that for now (it's not like I need to switch to PFNMAP);
adding vmf_insert_pfn_pmd() for when the remote side uses large pages,
it complains that the vmf->pmd is not a pmd_none nor huge nor a devmap
(this check appears specific to rhel7 kernel, I could temporarily test
with an upstream kernel but the network adapter won't work there so I'll
need this to work on this ultimately)

It looks like handle_mm_fault() will always try to allocate a pmd so it
should never be empty in my fault handler, and I don't see anything else
than vmf_insert_pfn_pmd() setting the mkdevmap flag, and it's not huge
either...
(on a dump, the the pmd content is 175cb18067, so these flags according
to crash for x86_64 are (PRESENT|RW|USER|ACCESSED|DIRTY))

I tried adding a huge_fault vm op thinking it might be called with a
more appropriate pmd but it doesn't seem to be called at all in my
case..? I would have assumed from the code that it would try every page

and if I try to somehow force it by using pmd_mkdevmap on the vmf->pmd,
things appear to work until the process exits and zap_page does a null
deref on pgtable_trans_huge_withdraw because the pgtable was never
deposited - this looks gone on newer kernels, but once again I do not
see where these should come from; I'm just assuming I reap what I sew
messing with the flags.



Long story short, I think I have some deeper undestanding problem about
the whole thing. Do I also need to use some specific flags when that
special file is mmap'd to allow huge_fault to be called ?
I think transparent_hugepage_enabled(vma) is fine, but the vmf.pmd found
in __handle_mm_fault is probably already not none at this point...?



Thanks again, feel free to ignore me for a bit longer I'll keep digging
my own grave, writing to a rubber duck that might have an idea of how
far the wrong way I've gone already helps... :D
-- 
Dominique