mm: swap: correctly use maxpages in swapon syscall to avoid potential deadloop

mm: swap: correctly use maxpages in swapon syscall to avoid potential deadloop

We use maxpages from read_swap_header() to initialize swap_info_struct,
however the maxpages might be reduced in setup_swap_extents() and the
si->max is assigned with the reduced maxpages from the
setup_swap_extents().

Obviously, this could lead to memory waste as we allocated memory based on
larger maxpages, besides, this could lead to a potential deadloop as
following:

1) When calling setup_clusters() with larger maxpages, unavailable
   pages within range [si->max, larger maxpages) are not accounted with
   inc_cluster_info_page().  As a result, these pages are assumed
   available but can not be allocated.  The cluster contains these pages
   can be moved to frag_clusters list after it's all available pages were
   allocated.

2) When the cluster mentioned in 1) is the only cluster in
   frag_clusters list, cluster_alloc_swap_entry() assume order 0
   allocation will never failed and will enter a deadloop by keep trying
   to allocate page from the only cluster in frag_clusters which contains
   no actually available page.

Call setup_swap_extents() to get the final maxpages before
swap_info_struct initialization to fix the issue.

After this change, span will include badblocks and will become large
value which I think is correct value:
In summary, there are two kinds of swapfile_activate operations.

1. Filesystem style: Treat all blocks logical continuity and find
   usable physical extents in logical range.  In this way, si->pages will
   be actual usable physical blocks and span will be "1 + highest_block -
   lowest_block".

2. Block device style: Treat all blocks physically continue and only
   one single extent is added.  In this way, si->pages will be si->max and
   span will be "si->pages - 1".  Actually, si->pages and si->max is only
   used in block device style and span value is set with si->pages.  As a
   result, span value in block device style will become a larger value as
   you mentioned.

I think larger value is correct based on:

1. Span value in filesystem style is "1 + highest_block -
   lowest_block" which is the range cover all possible phisical blocks
   including the badblocks.

2. For block device style, si->pages is the actual usable block number
   and is already in pr_info.  The original span value before this patch
   is also refer to usable block number which is redundant in pr_info.

[shikemeng@huaweicloud.com: ensure si->pages == si->max - 1 after setup_swap_extents()]
Link: https://lkml.kernel.org/r/20250522122554.12209-3-shikemeng@huaweicloud.com
Link: https://lkml.kernel.org/r/20250718065139.61989-1-shikemeng@huaweicloud.com
Link: https://lkml.kernel.org/r/20250522122554.12209-3-shikemeng@huaweicloud.com
Fixes: 661383c6111a ("mm: swap: relaim the cached parts that got scanned")
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>