Re: [RFC PATCH 1/2] mm: swap: check if swap backing device is congested or not

[Tim Chen]

On 12/18/18 3:43 PM, Yang Shi wrote:
> 
> 
> On 12/18/18 11:29 AM, Tim Chen wrote:
>> On 12/17/18 10:52 PM, Yang Shi wrote:
>>
>>> diff --git a/mm/swap_state.c b/mm/swap_state.c
>>> index fd2f21e..7cc3c29 100644
>>> --- a/mm/swap_state.c
>>> +++ b/mm/swap_state.c
>>> @@ -538,11 +538,15 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
>>> ÂÂÂÂÂ bool do_poll = true, page_allocated;
>>> ÂÂÂÂÂ struct vm_area_struct *vma = vmf->vma;
>>> ÂÂÂÂÂ unsigned long addr = vmf->address;
>>> +ÂÂÂ struct inode *inode = si->swap_file->f_mapping->host;
>>> Â ÂÂÂÂÂ mask = swapin_nr_pages(offset) - 1;
>>> ÂÂÂÂÂ if (!mask)
>>> ÂÂÂÂÂÂÂÂÂ goto skip;
>>> Â 
>> Shmem will also be using this function and I don't think the inode_read_congested
>> logic is relevant for that case.
> 
> IMHO, shmem is also relevant. As long as it is trying to readahead from swap, it should check if the underlying device is busy or not regardless of shmem or anon page.
> 

I don't think your dereference inode = si->swap_file->f_mapping->host
is always safe.  You should do it only when (si->flags & SWP_FS) is true.

Tim