mm: optimize truncation of shadow entries

Patch series "mm: optimize shadow entries removal", v2.

Some of our production workloads which processes a large amount of data
spends considerable amount of CPUs on truncation and invalidation of large
sized files (100s of GiBs of size).  Tracing the operations showed that
most of the time is in shadow entries removal.  This patch series
optimizes the truncation and invalidation operations.


This patch (of 2):

The kernel truncates the page cache in batches of PAGEVEC_SIZE.  For each
batch, it traverses the page cache tree and collects the entries (folio
and shadow entries) in the struct folio_batch.  For the shadow entries
present in the folio_batch, it has to traverse the page cache tree for
each individual entry to remove them.  This patch optimize this by
removing them in a single tree traversal.

On large machines in our production which run workloads manipulating large
amount of data, we have observed that a large amount of CPUs are spent on
truncation of very large files (100s of GiBs file sizes).  More
specifically most of time was spent on shadow entries cleanup, so
optimizing the shadow entries cleanup, even a little bit, has good impact.

To evaluate the changes, we created 200GiB file on a fuse fs and in a
memcg.  We created the shadow entries by triggering reclaim through
memory.reclaim in that specific memcg and measure the simple truncation
operation.

 # time truncate -s 0 file

              time (sec)
Without       5.164 +- 0.059
With-patch    4.21  +- 0.066 (18.47% decrease)

Link: https://lkml.kernel.org/r/20240925224716.2904498-1-shakeel.butt@linux.dev
Link: https://lkml.kernel.org/r/20240925224716.2904498-2-shakeel.butt@linux.dev


Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Chris Mason <clm@fb.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Omar Sandoval <osandov@osandov.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>