Re: [PATCH v2 3/3] mm,thp: add read-only THP support for (non-shmem) FS

[Song Liu <songliubraving@fb.com>]

Hi Kirill,

> On Jun 21, 2019, at 5:58 AM, Kirill A. Shutemov <kirill@shutemov.name> wrote:
> 
> On Fri, Jun 14, 2019 at 11:22:04AM -0700, Song Liu wrote:
>> This patch is (hopefully) the first step to enable THP for non-shmem
>> filesystems.
>> 
>> This patch enables an application to put part of its text sections to THP
>> via madvise, for example:
>> 
>>    madvise((void *)0x600000, 0x200000, MADV_HUGEPAGE);
>> 
>> We tried to reuse the logic for THP on tmpfs. The following functions are
>> renamed to reflect the new functionality:
>> 
>> 	collapse_shmem()	=>  collapse_file()
>> 	khugepaged_scan_shmem()	=>  khugepaged_scan_file()
>> 
>> Currently, write is not supported for non-shmem THP. This is enforced by
>> taking negative i_writecount. Therefore, if file has THP pages in the
>> page cache, open() to write will fail. To update/modify the file, the
>> user need to remove it first.
>> 
>> An EXPERIMENTAL config, READ_ONLY_THP_FOR_FS, is added to gate this
>> feature.
> 
> Please document explicitly that the feature opens local DoS attack: any
> user with read access to file can block write to the file by using
> MADV_HUGEPAGE for a range of the file.
> 
> As is it only has to be used with trusted userspace.
> 
> We also might want to have mount option in addition to Kconfig option to
> enable the feature on per-mount basis.

This behavior has been removed from v3 to v5. 

Thanks,
Song