linux-kernel - Re: [PATCH v2 4/4] mm: split underutilized THPs

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Message-ID: <3f6e1e0a-6132-4222-abb6-133224e11009@redhat.com>
Date: Fri, 9 Aug 2024 15:21:21 +0200
From: David Hildenbrand <david@...hat.com>
To: Usama Arif <usamaarif642@...il.com>, akpm@...ux-foundation.org,
 linux-mm@...ck.org
Cc: hannes@...xchg.org, riel@...riel.com, shakeel.butt@...ux.dev,
 roman.gushchin@...ux.dev, yuzhao@...gle.com, baohua@...nel.org,
 ryan.roberts@....com, rppt@...nel.org, willy@...radead.org,
 cerasuolodomenico@...il.com, corbet@....net, linux-kernel@...r.kernel.org,
 linux-doc@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH v2 4/4] mm: split underutilized THPs

On 09.08.24 12:31, Usama Arif wrote:
> 
> 
> On 08/08/2024 16:55, David Hildenbrand wrote:
>> On 07.08.24 15:46, Usama Arif wrote:
>>> This is an attempt to mitigate the issue of running out of memory when THP
>>> is always enabled. During runtime whenever a THP is being faulted in
>>> (__do_huge_pmd_anonymous_page) or collapsed by khugepaged
>>> (collapse_huge_page), the THP is added to  _deferred_list. Whenever memory
>>> reclaim happens in linux, the kernel runs the deferred_split
>>> shrinker which goes through the _deferred_list.
>>>
>>> If the folio was partially mapped, the shrinker attempts to split it.
>>> A new boolean is added to be able to distinguish between partially
>>> mapped folios and others in the deferred_list at split time in
>>> deferred_split_scan. Its needed as __folio_remove_rmap decrements
>>> the folio mapcount elements, hence it won't be possible to distinguish
>>> between partially mapped folios and others in deferred_split_scan
>>> without the boolean.
>>
>> Just so I get this right: Are you saying that we might now add fully mapped folios to the deferred split queue and that's what you want to distinguish?
> 
> Yes
> 
>>
>> If that's the case, then could we use a bit in folio->_flags_1 instead?
> Yes, thats a good idea. Will create the below flag for the next revision
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 5769fe6e4950..5825bd1cf6db 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -189,6 +189,11 @@ enum pageflags {
>   
>   #define PAGEFLAGS_MASK         ((1UL << NR_PAGEFLAGS) - 1)
>   
> +enum folioflags_1 {
> +       /* The first 8 bits of folio->_flags_1 are used to keep track of folio order */
> +       FOLIO_PARTIALLY_MAPPED = 8,     /* folio is partially mapped */
> +}

This might be what you want to achieve:

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index a0a29bd092f8..d4722ed60ef8 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -182,6 +182,7 @@ enum pageflags {
         /* At least one page in this folio has the hwpoison flag set */
         PG_has_hwpoisoned = PG_active,
         PG_large_rmappable = PG_workingset, /* anon or file-backed */
+       PG_partially_mapped, /* was identified to be partially mapped */
  };
  
  #define PAGEFLAGS_MASK         ((1UL << NR_PAGEFLAGS) - 1)
@@ -861,8 +862,9 @@ static inline void ClearPageCompound(struct page *page)
         ClearPageHead(page);
  }
  FOLIO_FLAG(large_rmappable, FOLIO_SECOND_PAGE)
+FOLIO_FLAG(partially_mapped, FOLIO_SECOND_PAGE)
  #else
-FOLIO_FLAG_FALSE(large_rmappable)
+FOLIO_FLAG_FALSE(partially_mapped)
  #endif
  
  #define PG_head_mask ((1UL << PG_head))

The downside is an atomic op to set/clear, but it should likely not really matter
(initially, the flag will be clear, and we should only ever set it once when
partially unmapping). If it hurts, we can reconsider.

[...]

>> I would actually suggest to split decoupling of "_deferred_list" and "partially mapped" into a separate preparation patch.
>>
> Yes, will do. I will split it into 3 patches, 1st one that introduces FOLIO_PARTIALLY_MAPPED and sets/clear it in the right place without introducing any functional change, 2nd to split underutilized THPs and 3rd to add sysfs entry to enable/disable the shrinker. Should make the patches quite small and easy to review.


Great! As always, please shout if you disagree with something I propose :)

-- 
Cheers,

David / dhildenb