Re: [PATCH v3 00/15] Free user PTE page table pages

[David Hildenbrand]

On 10.11.21 13:56, Jason Gunthorpe wrote:
> On Wed, Nov 10, 2021 at 06:54:13PM +0800, Qi Zheng wrote:
> 
>> In this patch series, we add a pte_refcount field to the struct page of page
>> table to track how many users of PTE page table. Similar to the mechanism of
>> page refcount, the user of PTE page table should hold a refcount to it before
>> accessing. The PTE page table page will be freed when the last refcount is
>> dropped.
> 
> So, this approach basically adds two atomics on every PTE map
> 
> If I have it right the reason that zap cannot clean the PTEs today is
> because zap cannot obtain the mmap lock due to a lock ordering issue
> with the inode lock vs mmap lock.

There are different ways to zap: madvise(DONTNEED) vs
fallocate(PUNCH_HOLE). It depends on "from where" we're actually
comming: a process page table walker or the rmap.

The way locking currently works doesn't allow to remove a page table
just by holding the mmap lock, not even in write mode. You'll also need
to hold the respective rmap locks -- which implies that reclaiming apge
tables crossing VMAs is "problematic". Take a look at khugepaged which
has to play quite some tricks to remove a page table.

And there are other ways we can create empty page tables via the rmap,
like reclaim/writeback, although they are rather a secondary concern mostly.

> 
> If it could obtain the mmap lock then it could do the zap using the
> write side as unmapping a vma does.
> 
> Rather than adding a new "lock" to ever PTE I wonder if it would be
> more efficient to break up the mmap lock and introduce a specific
> rwsem for the page table itself, in addition to the PTL. Currently the
> mmap lock is protecting both the vma list and the page table.

There is the rmap side of things as well. At least the rmap won't
reclaim alloc/free page tables, but it will walk page tables while
holding the respective rmap lock.

> 
> I think that would allow the lock ordering issue to be resolved and
> zap could obtain a page table rwsem.
> 
> Compared to two atomics per PTE this would just be two atomic per
> page table walk operation, it is conceptually a lot simpler, and would
> allow freeing all the page table levels, not just PTEs.

Another alternative is to not do it in the kernel automatically, but
instead have a madvise(MADV_CLEANUP_PGTABLE) mechanism that will get
called by user space explicitly once it's reasonable. While this will
work for the obvious madvise(DONTNEED) users -- like memory allocators
-- that zap memory, it's a bit more complicated once shared memory is
involved and we're fallocate(PUNCH_HOLE) memory. But it would at least
work for many use cases that want to optimize memory consumption for
sparse memory mappings.

Note that PTEs are the biggest memory consumer. On x86-64, a 1 TiB area
will consume 2 GiB of PTE tables and only 4 MiB of PMD tables. So PTEs
are most certainly the most important part piece.

-- 
Thanks,

David / dhildenb