Messages in this thread | | | Date | Wed, 10 Nov 2021 08:56:01 -0400 | From | Jason Gunthorpe <> | Subject | Re: [PATCH v3 00/15] Free user PTE page table pages |
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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.
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.
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.
?
Jason
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