On 2022-03-01 23:29, Miles Chen via iommu wrote:
Hi Yunfei,
Since __alloc_and_insert_iova_range fail will set the current alloc
iova size to max32_alloc_size (iovad->max32_alloc_size = size),
when the retry is executed into the __alloc_and_insert_iova_range
function, the retry action will be blocked by the check condition
(size >= iovad->max32_alloc_size) and goto iova32_full directly,
causes the action of retry regular alloc iova in
__alloc_and_insert_iova_range to not actually be executed.
Based on the above, so need reset max32_alloc_size before retry alloc
iova when alloc iova fail, that is set the initial dma_32bit_pfn value
of iovad to max32_alloc_size, so that the action of retry alloc iova
in __alloc_and_insert_iova_range can be executed.
Have you observed this making any difference in practice?
Given that both free_cpu_cached_iovas() and free_global_cached_iovas()
call iova_magazine_free_pfns(), which calls remove_iova(), which calls
__cached_rbnode_delete_update(), I'm thinking no...
Robin.
Like Robin pointed out, if some cached iovas are freed by
free_global_cached_iovas()/free_cpu_cached_iovas(),
the max32_alloc_size should be reset to iovad->dma_32bit_pfn.
If no cached iova is freed, resetting max32_alloc_size before
the retry allocation only give us a retry. Is it possible that
other users free their iovas during the additional retry?
No, it's not possible, since everyone's serialised by iova_rbtree_lock.
If the caches were already empty and the retry gets the lock first, it
will still fail again - forcing a reset of max32_alloc_size only means
it has to take the slow path to that failure. If another caller *did*
manage to get in and free something between free_global_cached_iovas()
dropping the lock and alloc_iova() re-taking it, then that would have
legitimately reset max32_alloc_size anyway.
Thanks,
Robin.
alloc_iova_fast()
retry:
alloc_iova() // failed, iovad->max32_alloc_size = size
free_cpu_cached_iovas()
iova_magazine_free_pfns()
remove_iova()
__cached_rbnode_delete_update()
iovad->max32_alloc_size = iovad->dma_32bit_pfn // reset
free_global_cached_iovas()
iova_magazine_free_pfns()
remove_iova()
__cached_rbnode_delete_update()
iovad->max32_alloc_size = iovad->dma_32bit_pfn // reset
goto retry;
thanks,
Miles
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