Re: [PATCH] mm: shmem: fix too little space for tmpfs only fallback 4KB

[Vernon Yang <vernon2gm@gmail.com>]

On Mon, Sep 22, 2025 at 09:46:53AM +0800, Baolin Wang wrote:
>
>
> On 2025/9/9 20:29, Vernon Yang wrote:
> >
> >
> > > On Sep 9, 2025, at 13:58, Baolin Wang <baolin.wang@linux.alibaba.com> wrote:
> > >
> > >
> > >
> > > On 2025/9/8 20:31, Vernon Yang wrote:
> > > > From: Vernon Yang <yanglincheng@kylinos.cn>
> > > > When the system memory is sufficient, allocating memory is always
> > > > successful, but when tmpfs size is low (e.g. 1MB), it falls back
> > > > directly from 2MB to 4KB, and other small granularity (8KB ~ 1024KB)
> > > > will not be tried.
> > > > Therefore add check whether the remaining space of tmpfs is sufficient
> > > > for allocation. If there is too little space left, try smaller large
> > > > folio.
> > >
> > > I don't think so.
> > >
> > > For a tmpfs mount with 'huge=within_size' and 'size=1M', if you try to write 1M data, it will allocate an order 8 large folio and will not fallback to order 0.
> > >
> > > For a tmpfs mount with 'huge=always' and 'size=1M', if you try to write 1M data, it will not completely fallback to order 0 either, instead, it will still allocate some order 1 to order 7 large folios.
> > >
> > > I'm not sure if this is your actual user scenario. If your files are small and you are concerned about not getting large folio allocations, I recommend using the 'huge=within_size' mount option.
> > >
> >
> > No, this is not my user scenario.
> >
> > Based on your previous patch [1], this scenario can be easily reproduced as
> > follows.
> >
> > $ mount -t tmpfs -o size=1024K,huge=always tmpfs /xxx/test
> > $ echo hello > /xxx/test/README
> > $ df -h
> > tmpfs            1.0M  4.0K 1020K   1% /xxx/test
> >
> > The code logic is as follows:
> >
> > shmem_get_folio_gfp()
> >      orders = shmem_allowable_huge_orders()
> >      shmem_alloc_and_add_folio(orders) return -ENOSPC;
> >          shmem_alloc_folio() alloc 2MB
> >          shmem_inode_acct_blocks()
> >              percpu_counter_limited_add() goto unacct;
> >          filemap_remove_folio()
> >      shmem_alloc_and_add_folio(order = 0)
> >
> >
> > As long as the tmpfs remaining space is too little and the system can allocate
> > memory 2MB, the above path will be triggered.
>
> In your scenario, wouldn't allocating 4K be more reasonable? Using a 1M
> large folio would waste memory. Moreover, if you want to use a large folio,
> I think you could increase the 'size' mount option. To me, this doesn't seem
> like a real-world usage scenario, instead it looks more like a contrived
> test case for a specific situation.

The previous example is just an easy demo to reproduce, and if someone
uses this example in the real world, of course the best method is to
increase the 'size'.

But the scenario I want to express here is that when the tmpfs space is
*consumed* to less than 2MB, only 4KB will be allocated, you can imagine
that when a tmpfs is constantly consumed, but someone is reclaiming or
freeing memory, causing often tmpfs space to remain in the range of
[0~2MB), then tmpfs will always only allocate 4KB.

> Sorry, this still doesn't convince me.