Re: [PATCH] mm: thp: Deny THP for guest_memfd and secretmem in file_thp_enabled()

["David Hildenbrand (Arm)" <david@kernel.org>]

>> BUT, something just occurred to me.
>>
>> We added the mc-handling in
>>
>> commit 98c76c9f1ef7599b39bfd4bd99b8a760d4a8cd3b
>> Author: Jiaqi Yan <jiaqiyan@google.com>
>> Date:   Wed Mar 29 08:11:19 2023 -0700
>>
>>       mm/khugepaged: recover from poisoned anonymous memory
>>
>> ..
>>
>> So I assume kernels before that would crash when collapsing?
>>
>> Looking at 5.15.199, it does not contain 98c76c9f1e [1].
>>
>> So I suspect we need a fix+stable backport.
>>
>> Who volunteers to try a secretmem reproducer on a stable kernel? :)
>>
> 
> I could give this a shot. 5.15.199 doesn't have AS_INACCESSIBLE. Should
> we backport AS_INACCESSIBLE there or could the fix for 5.15.199 just be
> special-casing secretmem like you suggested below?

Yes. If there is no guest_memfd we wouldn't need it.

> 
>>
>> The following is a bit nasty as well but should do the trick until we rip
>> out the CONFIG_READ_ONLY_THP_FOR_FS stuff.
>>
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 03886d4ccecc..4ac1cb36b861 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -40,6 +40,7 @@
>>    #include <linux/pgalloc.h>
>>    #include <linux/pgalloc_tag.h>
>>    #include <linux/pagewalk.h>
>> +#include <linux/secretmem.h>
>>
>>    #include <asm/tlb.h>
>>    #include "internal.h"
>> @@ -94,6 +95,10 @@ static inline bool file_thp_enabled(struct vm_area_struct *vma)
>>
>>           inode = file_inode(vma->vm_file);
>>
>> +       if (mapping_inaccessible(inode->i_mapping) ||
>> +           secretmem_mapping(inode->i_mapping))
>> +               return false;
>> +
> 
> Regarding the degradation of filesystems that don't support large folios
> yet: Do you mean having the collapse function respect AS_FOLIO_ORDER_MAX
> would disable collapsing for filesystems that actually want pages to be
> collapsed, but don't update max folio order and hence appear to not
> support large folios yet?
> 
> What about a check like this instead
> 
> 	if (!mapping_large_folio_support())
>          	return false;

That would essentially disable CONFIG_READ_ONLY_THP_FOR_FS (support for 
THP before filesystems started supporting large folios officially), no?

-- 
Cheers,

David