Re: [PATCH v3] mm: cma: indefinitely retry allocations in cma_alloc

[Chris Goldsworthy <cgoldswo@codeaurora.org>]

On 2020-09-27 12:23, Minchan Kim wrote:
> On Wed, Sep 23, 2020 at 10:16:25PM -0700, Chris Goldsworthy wrote:
>> CMA allocations will fail if 'pinned' pages are in a CMA area, since 
>> we


>> 
>> +config CMA_RETRY_SLEEP_DURATION
>> +	int "Sleep duration between retries"
>> +	depends on CMA
>> +	default 100
>> +	help
>> +	  Time to sleep for in milliseconds between the indefinite
>> +	  retries of a CMA allocation that are induced by passing
>> +	  __GFP_NOFAIL to cma_alloc().
>> +
>> +	  If unsure, leave the value as "100".
> 
> What's the point of this new config? If we need it, How could admin
> set their value?
> How does it make bad if we just use simple default vaule?
> IOW, I'd like to avoid introducing new config if we don't see good
> justifcation.

I thought that it would be useful for developers, but I guess it would 
be much better for this to be runtime configurable.  But, I don't think 
there's a strong reason to be able to configure the value - 100 ms has 
worked for us.  I'll update scrap this option in a follow-up patch, and 
will use 100 ms as the sleeping time.

>> +
>>  config MEM_SOFT_DIRTY
>>  	bool "Track memory changes"
>>  	depends on CHECKPOINT_RESTORE && HAVE_ARCH_SOFT_DIRTY && PROC_FS
>> diff --git a/mm/cma.c b/mm/cma.c
>> index 7f415d7..4fbad2b 100644
>> --- a/mm/cma.c
>> +++ b/mm/cma.c
>> @@ -32,6 +32,7 @@
>>  #include <linux/highmem.h>
>>  #include <linux/io.h>
>>  #include <linux/kmemleak.h>
>> +#include <linux/delay.h>
>>  #include <trace/events/cma.h>
>> 
>>  #include "cma.h"
>> @@ -403,13 +404,15 @@ static inline void cma_debug_show_areas(struct 
>> cma *cma) { }
>>   * @cma:   Contiguous memory region for which the allocation is 
>> performed.
>>   * @count: Requested number of pages.
>>   * @align: Requested alignment of pages (in PAGE_SIZE order).
>> - * @no_warn: Avoid printing message about failed allocation
>> + * @gfp_mask: If __GFP_NOWARN is passed, suppress messages about 
>> failed
>> + *	      allocations. If __GFP_NOFAIL is passed, try doing the CMA
>> + *	      allocation indefinitely until the allocation succeeds.
>>   *
>>   * This function allocates part of contiguous memory on specific
>>   * contiguous memory area.
>>   */
>>  struct page *cma_alloc(struct cma *cma, size_t count, unsigned int 
>> align,
>> -		       bool no_warn)
>> +		       gfp_t gfp_mask)
>>  {
>>  	unsigned long mask, offset;
>>  	unsigned long pfn = -1;
>> @@ -442,8 +445,28 @@ struct page *cma_alloc(struct cma *cma, size_t 
>> count, unsigned int align,
>>  				bitmap_maxno, start, bitmap_count, mask,
>>  				offset);
>>  		if (bitmap_no >= bitmap_maxno) {
>> -			mutex_unlock(&cma->lock);
>> -			break;
>> +			if (ret == -EBUSY && gfp_mask & __GFP_NOFAIL) {
>> +				mutex_unlock(&cma->lock);
>> +
>> +				/*
>> +				 * Page may be momentarily pinned by some other
>> +				 * process which has been scheduled out, e.g.
>> +				 * in exit path, during unmap call, or process
>> +				 * fork and so cannot be freed there. Sleep
>> +				 * for 100ms and retry the allocation.
>> +				 */
>> +				start = 0;
>> +				ret = -ENOMEM;
>> +				msleep(CONFIG_CMA_RETRY_SLEEP_DURATION);
> 
> Should it be uninterruptible, really?

Good point - I'll replace the msleep() this with 
schedule_timeout_killable() in the follow-up patch.

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