Re: [v3 PATCH 1/2] mm: swap: check if swap backing device is congested or not

[Yang Shi]

On 12/21/18 2:42 PM, Tim Chen wrote:
> On 12/21/18 1:40 PM, Yang Shi wrote:
> > Swap readahead would read in a few pages regardless if the underlying
> > device is busy or not.  It may incur long waiting time if the device is
> > congested, and it may also exacerbate the congestion.
> >
> > Use inode_read_congested() to check if the underlying device is busy or
> > not like what file page readahead does.  Get inode from swap_info_struct.
> > Although we can add inode information in swap_address_space
> > (address_space->host), it may lead some unexpected side effect, i.e.
> > it may break mapping_cap_account_dirty().  Using inode from
> > swap_info_struct seems simple and good enough.
> >
> > Just does the check in vma_cluster_readahead() since
> > swap_vma_readahead() is just used for non-rotational device which
> > much less likely has congestion than traditional HDD.
> >
> > Although swap slots may be consecutive on swap partition, it still may be
> > fragmented on swap file. This check would help to reduce excessive stall
> > for such case.
> >
> > Cc: Huang Ying <ying.huang@xxxxxxxxx>
> > Cc: Tim Chen <tim.c.chen@xxxxxxxxx>
> > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
> > ---
> > v3: Move inode deference under swap device type check per Tim Chen
> > v2: Check the swap device type per Tim Chen
> >
> >   mm/swap_state.c | 7 +++++++
> >   1 file changed, 7 insertions(+)
> >
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index fd2f21e..78d500e 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -538,11 +538,18 @@ struct page *swap_cluster_readahead(swp_entry_t entry, gfp_t gfp_mask,
> >   	bool do_poll = true, page_allocated;
> >   	struct vm_area_struct *vma = vmf->vma;
> >   	unsigned long addr = vmf->address;
> > +	struct inode *inode = NULL;
> >   
> >   	mask = swapin_nr_pages(offset) - 1;
> >   	if (!mask)
> >   		goto skip;
> >   
> > +	if (si->flags & (SWP_BLKDEV | SWP_FS)) {
> > +		inode = si->swap_file->f_mapping->host;
> > +		if (inode_read_congested(inode))
> > +			goto skip;
> > +	}
> > +
> >   	do_poll = false;
> >   	/* Read a page_cluster sized and aligned cluster around offset. */
> >   	start_offset = offset & ~mask;
> Acked-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>

Thanks. Happy holiday.