Re: [PATCH RFC 04/10] mm: page_frag: introduce page_frag_alloc_abort() related API

[Yunsheng Lin <linyunsheng@huawei.com>]

On 2024/10/29 1:53, Alexander Duyck wrote:
> On Mon, Oct 28, 2024 at 5:05 AM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>>
>> For some case as tun_build_skb() without the needing of
>> using complicated prepare & commit API, add the abort API to
>> abort the operation of page_frag_alloc_*() related API for
>> error handling knowing that no one else is taking extra
>> reference to the just allocated fragment, and add abort_ref
>> API to only abort the reference counting of the allocated
>> fragment if it is already referenced by someone else.
>>

...

>> +
>> +/**
>> + * page_frag_alloc_abort_ref - Abort the reference of allocated fragment.
>> + * @nc: page_frag cache to which the page fragment is aborted back
>> + * @va: virtual address of page fragment to be aborted
>> + * @fragsz: size of the page fragment to be aborted
>> + *
>> + * It is expected to be called from the same context as the allocation API.
>> + * Mostly used for error handling cases to abort the reference of allocated
>> + * fragment if the fragment has been referenced for other usages, to aovid the
>> + * atomic operation of page_frag_free() API.
>> + */
>> +void page_frag_alloc_abort_ref(struct page_frag_cache *nc, void *va,
>> +                              unsigned int fragsz)
>> +{
>> +       VM_BUG_ON(va + fragsz !=
>> +                 encoded_page_decode_virt(nc->encoded_page) + nc->offset);
>> +
>> +       nc->pagecnt_bias++;
>> +}
>> +EXPORT_SYMBOL(page_frag_alloc_abort_ref);
> 
> It isn't clear to me why you split this over two functions. It seems
> like you could just update the offset in this lower function rather
> than do it in the upper one since you are passing all the arguments
> here anyway.

For the usecase in tun_build_skb(), There seems to be XDP_REDIRECT and
XDP_TX case that the allocated fragment has been referenced for other
usages even when xdp_do_redirect() or tun_xdp_tx() return error, so that
caller can call page_frag_alloc_abort_ref() to abort its reference
of the allocated fragment, but not abort the whole fragment for later
reuse.

As the doc mentioned above page_frag_alloc_abort_ref(), it is mainly to
avoid the atomic operation of page_frag_free() API when the caller has the
allocation context and has the all the arguments page_frag_alloc_abort_ref()
needs even though it might be a unlikely case if page_frag_alloc_abort() API
is already provided.