Re: [PATCH v9 04/10] mm: thp: Support allocation of anonymous multi-size THP

[Ryan Roberts <ryan.roberts@arm.com>]

On 13/12/2023 07:21, Dan Carpenter wrote:
> On Thu, Dec 07, 2023 at 04:12:05PM +0000, Ryan Roberts wrote:
>> @@ -4176,10 +4260,15 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>>  	/* Allocate our own private page. */
>>  	if (unlikely(anon_vma_prepare(vma)))
>>  		goto oom;
>> -	folio = vma_alloc_zeroed_movable_folio(vma, vmf->address);
>> +	folio = alloc_anon_folio(vmf);
>> +	if (IS_ERR(folio))
>> +		return 0;
>>  	if (!folio)
>>  		goto oom;
> 
> Returning zero is weird.  I think it should be a vm_fault_t code.

It's the same pattern that the existing code a little further down this function
already implements:

	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, addr, &vmf->ptl);
	if (!vmf->pte)
		goto release;

If we fail to map/lock the pte (due to a race), then we return 0 to allow user
space to rerun the faulting instruction and cause the fault to happen again. The
above code ends up calling "return ret;" and ret is 0.

> 
> This mixing of error pointers and NULL is going to cause problems.
> Normally when we have a mix of error pointers and NULL then the NULL is
> not an error but instead means that the feature has been deliberately
> turned off.  I'm unable to figure out what the meaning is here.

There are 3 conditions that the function can return:

 - folio successfully allocated
 - folio failed to be allocated due to OOM
 - fault needs to be tried again due to losing race

Previously only the first 2 conditions were possible and they were indicated by
NULL/not-NULL. The new 3rd condition is only possible when THP is compile-time
enabled. So it keeps the logic simpler to keep the NULL/not-NULL distinction for
the first 2, and use the error code for the final one.

There are IS_ERR() and IS_ERR_OR_NULL() variants so I assume a pattern where you
can have pointer, error or NULL is somewhat common already?

Thanks,
Ryan

> 
> It should return one or the other, or if it's a mix then add a giant
> comment explaining what they mean.
> 
> regards,
> dan carpenter
> 
>>  
>> +	nr_pages = folio_nr_pages(folio);
>> +	addr = ALIGN_DOWN(vmf->address, nr_pages * PAGE_SIZE);
>> +
>>  	if (mem_cgroup_charge(folio, vma->vm_mm, GFP_KERNEL))
>>  		goto oom_free_page;
>>  	folio_throttle_swaprate(folio, GFP_KERNEL);
>