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    SubjectRe: [PATCH -mm -v4 04/21] mm, THP, swap: Support PMD swap mapping in swapcache_free_cluster()
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    Date
    > +#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?

    > /*
    > * 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.

    > @@ -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;

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