Re: [LSF/MM/BPF TOPIC] Removing GFP_NOFS

Vlastimil Babka (SUSE)
2024-01-05T14:35:50Z

On 1/5/24 11:13, Viacheslav Dubeyko wrote:
>
>> On Jan 5, 2024, at 12:17 AM, Matthew Wilcox <willy@infradead.org> wrote:
>>
>> The memalloc_nofs APIs were introduced in May 2017, but we still have
>> over 1000 uses of GFP_NOFS in fs/ today (and 200 outside fs/, which is
>> really sad). This session is for filesystem developers to talk about
>> what they need to do to fix up their own filesystem, or share stories
>> about how they made their filesystem better by adopting the new APIs.
>>
>
> Many file systems are still heavily using GFP_NOFS for kmalloc and
> kmem_cache_alloc family methods even if memalloc_nofs_save() and
> memalloc_nofs_restore() pair is used too. But I can see that GFP_NOFS

Yes it should be enough to rely on memalloc_nofs_save() for
kmalloc/kmem_cache_alloc. The kmalloc layer doesnt't care about it, and once
it's run out of available slab folios and calls into the page allocator for
a new one, it evaluates the effect of memalloc_nofs_save() as expected.

> is used in radix_tree_preload(), bio_alloc(), posix_acl_clone(),
> sb_issue_zeroout, sb_issue_discard(), alloc_inode_sb(), blkdev_issue_zeroout(),
> blkdev_issue_secure_erase(), blkdev_zone_mgmt(), etc.
>
> Would it be safe to switch on memalloc_nofs_save()/memalloc_nofs_restore() for
> all possible cases? Any potential issues or downsides?
>
> Thanks,
> Slava.
>
>