linux-kernel - Re: [PATCH 07/12] huge_memory: Allow mappings of PMD sized pages

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Message-ID: <875xpicsbd.fsf@nvdebian.thelocal>
Date: Thu, 24 Oct 2024 10:14:18 +1100
From: Alistair Popple <apopple@...dia.com>
To: Alistair Popple <apopple@...dia.com>
Cc: Dan Williams <dan.j.williams@...el.com>, linux-mm@...ck.org,
 vishal.l.verma@...el.com, dave.jiang@...el.com, logang@...tatee.com,
 bhelgaas@...gle.com, jack@...e.cz, jgg@...pe.ca, catalin.marinas@....com,
 will@...nel.org, mpe@...erman.id.au, npiggin@...il.com,
 dave.hansen@...ux.intel.com, ira.weiny@...el.com, willy@...radead.org,
 djwong@...nel.org, tytso@....edu, linmiaohe@...wei.com, david@...hat.com,
 peterx@...hat.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, nvdimm@...ts.linux.dev,
 linux-cxl@...r.kernel.org, linux-fsdevel@...r.kernel.org,
 linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
 jhubbard@...dia.com, hch@....de, david@...morbit.com
Subject: Re: [PATCH 07/12] huge_memory: Allow mappings of PMD sized pages

Alistair Popple <apopple@...dia.com> writes:

> Alistair Popple wrote:
>> Dan Williams <dan.j.williams@...el.com> writes:

[...]

>>> +
>>> +	return VM_FAULT_NOPAGE;
>>> +}
>>> +EXPORT_SYMBOL_GPL(dax_insert_pfn_pmd);
>>
>> Like I mentioned before, lets make the exported function
>> vmf_insert_folio() and move the pte, pmd, pud internal private / static
>> details of the implementation. The "dax_" specific aspect of this was
>> removed at the conversion of a dax_pfn to a folio.
>
> Ok, let me try that. Note that vmf_insert_pfn{_pmd|_pud} will have to
> stick around though.

Creating a single vmf_insert_folio() seems somewhat difficult because it
needs to be called from multiple fault paths (either PTE, PMD or PUD
fault) and do something different for each.

Specifically the issue I ran into is that DAX does not downgrade PMD
entries to PTE entries if they are backed by storage. So the PTE fault
handler will get a PMD-sized DAX entry and therefore a PMD size folio.

The way I tried implementing vmf_insert_folio() was to look at
folio_order() to determine which internal implementation to call. But
that doesn't work for a PTE fault, because there's no way to determine
if we should PTE map a subpage or PMD map the entire folio.

We could pass down some context as to what type of fault we're handling,
or add it to the vmf struct, but that seems excessive given callers
already know this and could just call a specific
vmf_insert_page_{pte|pmd|pud}.