Re: [PATCH] mm: shmem: fix too little space for tmpfs only fallback 4KB
From: Baolin Wang
Date: Sun Sep 21 2025 - 21:47:02 EST
On 2025/9/9 20:29, Vernon Yang wrote:
On Sep 9, 2025, at 13:58, Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx> wrote:
On 2025/9/8 20:31, Vernon Yang wrote:
From: Vernon Yang <yanglincheng@xxxxxxxxxx>
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.
Sorry, this still doesn't convince me.