Re: [PATCH -mm -v4 04/21] mm, THP, swap: Support PMD swap mapping in swapcache_free_cluster()

Huang, Ying
2018-07-10T06:53:44Z

Dave Hansen <dave.hansen@linux.intel.com> writes:

>> +#ifdef CONFIG_THP_SWAP
>> +static inline int cluster_swapcount(struct swap_cluster_info *ci)
>> +{
>> + if (!ci || !cluster_is_huge(ci))
>> + return 0;
>> +
>> + return cluster_count(ci) - SWAPFILE_CLUSTER;
>> +}
>> +#else
>> +#define cluster_swapcount(ci) 0
>> +#endif
>
> Dumb questions, round 2: On a CONFIG_THP_SWAP=n build, presumably,
> cluster_is_huge()=0 always, so cluster_swapout() always returns 0. Right?
>
> So, why the #ifdef?

#ifdef here is to reduce the code size for !CONFIG_THP_SWAP.

>> /*
>> * It's possible scan_swap_map() uses a free cluster in the middle of free
>> * cluster list. Avoiding such abuse to avoid list corruption.
>> @@ -905,6 +917,7 @@ static void swap_free_cluster(struct swap_info_struct *si, unsigned long idx)
>> struct swap_cluster_info *ci;
>>
>> ci = lock_cluster(si, offset);
>> + memset(si->swap_map + offset, 0, SWAPFILE_CLUSTER);
>> cluster_set_count_flag(ci, 0, 0);
>> free_cluster(si, idx);
>> unlock_cluster(ci);
>
> This is another case of gloriously comment-free code, but stuff that
> _was_ covered in the changelog. I'd much rather have code comments than
> changelog comments. Could we fix that?
>
> I'm generally finding it quite hard to review this because I keep having
> to refer back to the changelog to see if what you are doing matches what
> you said you were doing.

Sure. Will fix this.

>> @@ -1288,24 +1301,30 @@ static void swapcache_free_cluster(swp_entry_t entry)
>>
>> ci = lock_cluster(si, offset);
>> VM_BUG_ON(!cluster_is_huge(ci));
>> + VM_BUG_ON(!is_cluster_offset(offset));
>> + VM_BUG_ON(cluster_count(ci) < SWAPFILE_CLUSTER);
>> map = si->swap_map + offset;
>> - for (i = 0; i < SWAPFILE_CLUSTER; i++) {
>> - val = map[i];
>> - VM_BUG_ON(!(val & SWAP_HAS_CACHE));
>> - if (val == SWAP_HAS_CACHE)
>> - free_entries++;
>> + if (!cluster_swapcount(ci)) {
>> + for (i = 0; i < SWAPFILE_CLUSTER; i++) {
>> + val = map[i];
>> + VM_BUG_ON(!(val & SWAP_HAS_CACHE));
>> + if (val == SWAP_HAS_CACHE)
>> + free_entries++;
>> + }
>> + if (free_entries != SWAPFILE_CLUSTER)
>> + cluster_clear_huge(ci);
>> }
>
> Also, I'll point out that cluster_swapcount() continues the horrific
> naming of cluster_couunt(), not saying what the count is *of*. The
> return value doesn't help much:
>
> return cluster_count(ci) - SWAPFILE_CLUSTER;

We have page_swapcount() for page, swp_swapcount() for swap entry.
cluster_swapcount() tries to mimic them for swap cluster. But I am not
good at naming in general. What's your suggestion?

Best Regards,
Huang, Ying