Le 15/01/2024 à 19:37, Jason Gunthorpe a écrit :

On Wed, Jan 03, 2024 at 05:14:16PM +0800, peterx@redhat.com wrote:

quoted

From: Peter Xu <peterx@redhat.com>

Hugepd format for GUP is only used in PowerPC with hugetlbfs.  There are
some kernel usage of hugepd (can refer to hugepd_populate_kernel() for
PPC_8XX), however those pages are not candidates for GUP.

Commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
file-backed mappings") added a check to fail gup-fast if there's potential
risk of violating GUP over writeback file systems.  That should never apply
to hugepd.  Considering that hugepd is an old format (and even
software-only), there's no plan to extend hugepd into other file typed
memories that is prone to the same issue.

I didn't dig into the ppc stuff too deeply, but this looks to me like
it is the same thing as ARM's contig bits?

ie a chunk of PMD/etc entries are all managed together as though they
are a virtual larger entry and we use the hugepte_addr_end() stuff to
iterate over each sub entry.

As far as I understand ARM's contig stuff, hugepd on powerpc is 
something different.

hugepd is a page directory dedicated to huge pages, where you have huge 
pages listed instead of regular pages. For instance, on powerpc 32 with 
each PGD entries covering 4Mbytes, a regular page table has 1024 PTEs. A 
hugepd for 512k is a page table with 8 entries.

And for 8Mbytes entries, the hugepd is a page table with only one entry. 
And 2 consecutive PGS entries will point to the same hugepd to cover the 
entire 8Mbytes.

But WHY is GUP doing this or caring about this? GUP should have no
problem handling the super-size entry (eg 8M on nohash) as a single
thing. It seems we only lack an API to get this out of the arch code?

It seems to me we should see ARM and PPC agree on what the API is for
this and then get rid of hugepd by making both use the same page table
walker API. Is that too hopeful?

Can't see the similarity between ARM contig PTE and PPC huge page 
directories.

quoted

Drop that check, not only because it'll never be true for hugepd per any
known plan, but also it paves way for reusing the function outside
fast-gup.

I didn't see any other caller of this function in this series? When
does this re-use happen??

Jason

Christophe