Re: [PATCH v2] mm: alloc_pages_bulk: remove assumption of populating only NULL elements

[Qu Wenruo]

在 2025/2/28 20:14, Yunsheng Lin 写道:
> As mentioned in [1], it seems odd to check NULL elements in
> the middle of page bulk allocating, and it seems caller can
> do a better job of bulk allocating pages into a whole array
> sequentially without checking NULL elements first before
> doing the page bulk allocation for most of existing users.
>
> Through analyzing of bulk allocation API used in fs, it
> seems that the callers are depending on the assumption of
> populating only NULL elements in fs/btrfs/extent_io.c and
> net/sunrpc/svc_xprt.c while erofs and btrfs don't, see:
> commit 91d6ac1d62c3 ("btrfs: allocate page arrays using bulk page allocator")

If you want to change the btrfs part, please run full fstests with 
SCRATCH_DEV_POOL populated at least.

[...]
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index f0a1da40d641..ef52cedd9873 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -623,13 +623,26 @@ int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array,
>   			   bool nofail)
>   {
>   	const gfp_t gfp = nofail ? (GFP_NOFS | __GFP_NOFAIL) : GFP_NOFS;
> -	unsigned int allocated;
> +	unsigned int allocated, ret;
>   
> -	for (allocated = 0; allocated < nr_pages;) {
> -		unsigned int last = allocated;
> +	/* Defragment page_array so pages can be bulk allocated into remaining
> +	 * NULL elements sequentially.
> +	 */
> +	for (allocated = 0, ret = 0; ret < nr_pages; ret++) {
> +		if (page_array[ret]) {

You just prove how bad the design is.

All the callers have their page array members to initialized to NULL, or 
do not care and just want alloc_pages_bulk() to overwrite the 
uninitialized values.

The best example here is btrfs_encoded_read_regular().
Now your code will just crash encoded read.

Read the context before doing stupid things.

I find it unacceptable that you just change the code, without any 
testing, nor even just check all the involved callers.

> +			page_array[allocated] = page_array[ret];
> +			if (ret != allocated)
> +				page_array[ret] = NULL;
> +
> +			allocated++;
> +		}
> +	}
>   
> -		allocated = alloc_pages_bulk(gfp, nr_pages, page_array);
> -		if (unlikely(allocated == last)) {
> +	while (allocated < nr_pages) {
> +		ret = alloc_pages_bulk(gfp, nr_pages - allocated,
> +				       page_array + allocated);

I see the new interface way worse than the existing one.

All btrfs usage only wants a simple retry-until-all-fulfilled behavior.

NACK for btrfs part, and I find you very unresponsible not even bother 
running any testsuit and just submit such a mess.

Just stop this, no one will ever take you serious anymore.

Thanks,
Qu