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

[Christoph Hellwig <hch@infradead.org>]

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)

> --- 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.