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

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 11/10/21 9:25 PM, David Hildenbrand wrote:
> 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.
> 

total agree!

Thanks,
Qi