Re: [RFC PATCH 1/8] mm: Provide pagesize to pmd_populate()

Le 25/03/2024 à 17:19, Jason Gunthorpe a écrit :

On Mon, Mar 25, 2024 at 03:55:54PM +0100, Christophe Leroy wrote:

quoted

Unlike many architectures, powerpc 8xx hardware tablewalk requires
a two level process for all page sizes, allthough second level only
has one entry when pagesize is 8M.

To fit with Linux page table topology and without requiring special
page directory layout like hugepd, the page entry will be replicated
1024 times in the standard page table. However for large pages it is
necessary to set bits in the level-1 (PMD) entry. At the time being,
for 512k pages the flag is kept in the PTE and inserted in the PMD
entry at TLB miss exception, that is necessary because we can have
pages of different sizes in a page table. However the 12 PTE bits are
fully used and there is no room for an additional bit for page size.

For 8M pages, there will be only one page per PMD entry, it is
therefore possible to flag the pagesize in the PMD entry, with the
advantage that the information will already be at the right place for
the hardware.

To do so, add a new helper called pmd_populate_size() which takes the
page size as an additional argument, and modify __pte_alloc() to also
take that argument. pte_alloc() is left unmodified in order to
reduce churn on callers, and a pte_alloc_size() is added for use by
pte_alloc_huge().

When an architecture doesn't provide pmd_populate_size(),
pmd_populate() is used as a fallback.

I think it would be a good idea to document what the semantic is
supposed to be for sz?

Just a general remark, probably nothing for this, but with these new
arguments the historical naming seems pretty tortured for
pte_alloc_size().. Something like pmd_populate_leaf(size) as a naming
scheme would make this more intuitive. Ie pmd_populate_leaf() gives
you a PMD entry where the entry points to a leaf page table able to
store folios of at least size.

Anyhow, I thought the edits to the mm helpers were fine, certainly
much nicer than hugepd. Do you see a path to remove hugepd entirely
from here?

Not looked into details yet, but I guess so.

By the way there is a wiki dedicated to huge pages on powerpc, you can 
have a look at it here : 
https://github.com/linuxppc/wiki/wiki/Huge-pages , maybe you'll find 
good ideas there to help me.

Christophe