linux-kernel - Re: [PATCH] mm/huge_memory: Avoid PMD-size page cache if needed

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Message-ID: <f58433ee-7217-4f9e-91ba-c29f95cd56b0@redhat.com>
Date: Thu, 11 Jul 2024 23:03:11 +0200
From: David Hildenbrand <david@...hat.com>
To: Matthew Wilcox <willy@...radead.org>, Gavin Shan <gshan@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 akpm@...ux-foundation.org, william.kucharski@...cle.com,
 ryan.roberts@....com, shan.gavin@...il.com
Subject: Re: [PATCH] mm/huge_memory: Avoid PMD-size page cache if needed

On 11.07.24 22:46, Matthew Wilcox wrote:
> On Thu, Jul 11, 2024 at 08:48:40PM +1000, Gavin Shan wrote:
>> +++ b/mm/huge_memory.c
>> @@ -136,7 +136,8 @@ unsigned long __thp_vma_allowable_orders(struct vm_area_struct *vma,
>>   
>>   		while (orders) {
>>   			addr = vma->vm_end - (PAGE_SIZE << order);
>> -			if (thp_vma_suitable_order(vma, addr, order))
>> +			if (!(vma->vm_file && order > MAX_PAGECACHE_ORDER) &&
>> +			    thp_vma_suitable_order(vma, addr, order))
>>   				break;
> 
> Why does 'orders' even contain potential orders that are larger than
> MAX_PAGECACHE_ORDER?
> 
> We do this at the top:
> 
>          orders &= vma_is_anonymous(vma) ?
>                          THP_ORDERS_ALL_ANON : THP_ORDERS_ALL_FILE;
> 
> include/linux/huge_mm.h:#define THP_ORDERS_ALL_FILE     (BIT(PMD_ORDER) | BIT(PUD_ORDER))
> 
> ... and that seems very wrong.  We support all kinds of orders for
> files, not just PMD order.  We don't support PUD order at all.
> 
> What the hell is going on here?

yes, that's just absolutely confusing. I mentioned it to Ryan lately 
that we should clean that up (I wanted to look into that, but am happy 
if someone else can help).

There should likely be different defines for

DAX (PMD|PUD)

SHMEM (PMD) -- but soon more. Not sure if we want separate ANON_SHMEM 
for the time being. Hm. But shmem is already handles separately, so 
maybe we can just ignore shmem here.

PAGECACHE (1 .. MAX_PAGECACHE_ORDER)

? But it's still unclear to me.

At least DAX must stay special I think, and PAGECACHE should be capped 
at MAX_PAGECACHE_ORDER.

-- 
Cheers,

David / dhildenb