Re: [PATCH v4] mm: cma: indefinitely retry allocations in cma_alloc

[Chris Goldsworthy <cgoldswo@codeaurora.org>]

On 2020-09-28 22:59, Christoph Hellwig wrote:
> On Mon, Sep 28, 2020 at 01:30:27PM -0700, Chris Goldsworthy wrote:
>> CMA allocations will fail if 'pinned' pages are in a CMA area, since 
>> we
>> cannot migrate pinned pages. The _refcount of a struct page being 
>> greater
>> than _mapcount for that page can cause pinning for anonymous pages.  
>> This
>> is because try_to_unmap(), which (1) is called in the CMA allocation 
>> path,
>> and (2) decrements both _refcount and _mapcount for a page, will stop
>> unmapping a page from VMAs once the _mapcount for a page reaches 0.  
>> This
>> implies that after try_to_unmap() has finished successfully for a page
>> where _recount > _mapcount, that _refcount will be greater than 0.  
>> Later
>> in the CMA allocation path in migrate_page_move_mapping(), we will 
>> have one
>> more reference count than intended for anonymous pages, meaning the
>> allocation will fail for that page.
>> 
>> If a process ends up causing _refcount > _mapcount for a page (by 
>> either
>> incrementing _recount or decrementing _mapcount), such that the 
>> process is
>> context switched out after modifying one refcount but before modifying 
>> the
>> other, the page will be temporarily pinned.
>> 
>> One example of where _refcount can be greater than _mapcount is inside 
>> of
>> zap_pte_range(), which is called for all the entries of a PMD when a
>> process is exiting, to unmap the process's memory.  Inside of
>> zap_pte_range(), after unammping a page with page_remove_rmap(), we 
>> have
>> that _recount > _mapcount.  _refcount can only be decremented after a 
>> TLB
>> flush is performed for the page - this doesn't occur until enough 
>> pages
>> have been batched together for flushing.  The flush can either occur 
>> inside
>> of zap_pte_range() (during the same invocation or a later one), or if 
>> there
>> aren't enough pages collected by the time we unmap all of the pages in 
>> a
>> process, the flush will occur in tlb_finish_mmu() in exit_mmap().  
>> After
>> the flush has occurred, tlb_batch_pages_flush() will decrement the
>> references on the flushed pages.
>> 
>> Another such example like the above is inside of copy_one_pte(), which 
>> is
>> called during a fork. For PTEs for which pte_present(pte) == true,
>> copy_one_pte() will increment the _refcount field followed by the
>> _mapcount field of a page.
>> 
>> So, inside of cma_alloc(), add the option of letting users pass in
>> __GFP_NOFAIL to indicate that we should retry CMA allocations 
>> indefinitely,
>> in the event that alloc_contig_range() returns -EBUSY after having 
>> scanned
>> a whole CMA-region bitmap.
> 
> And who is going to use this?  AS-is this just seems to add code that
> isn't actually used and thus actually tested.  (In addition to beeing
> a relly bad idea as discussed before)

Hi Christoph,

That had slipped my mind - what we would have submitted would have been 
a modified /drivers/dma-heap/heaps/cma_heap.c, which would have created 
a "linux,cma-nofail" heap, that when allocated from, passes GFP_NOFAIL 
to cma_alloc().  But, since this retry approach (finite and infinite) 
has effectively been nacked, I've gone back to the drawing board to find 
either (1) a lock based approach to solving this (as posed by Andrew 
Morton here: https://lkml.org/lkml/2020/8/21/1490), or (2) using 
preempt_disable() calls.

Thanks,

Chris.

>> --- a/kernel/dma/contiguous.c
>> +++ b/kernel/dma/contiguous.c
>> @@ -196,7 +196,7 @@ struct page *dma_alloc_from_contiguous(struct 
>> device *dev, size_t count,
>>  	if (align > CONFIG_CMA_ALIGNMENT)
>>  		align = CONFIG_CMA_ALIGNMENT;
>> 
>> -	return cma_alloc(dev_get_cma_area(dev), count, align, no_warn);
>> +	return cma_alloc(dev_get_cma_area(dev), count, align, no_warn ? 
>> __GFP_NOWARN : 0);
> 
> Also don't add pointlessly overlong lines.

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