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When system is under memory pressure it is observed that dm bufio shrinker often reclaims only one buffer per scan. This change fixes the following two issues in dm bufio shrinker that cause this behavior: 1. ((nr_to_scan - freed) <= retain_target) condition is used to terminate slab scan process. This assumes that nr_to_scan is equal to the LRU size, which might not be correct because do_shrink_slab() in vmscan.c calculates nr_to_scan using multiple inputs. As a result when nr_to_scan is less than retain_target (64) the scan will terminate after the first iteration, effectively reclaiming one buffer per scan and making scans very inefficient. This hurts vmscan performance especially because mutex is acquired/released every time dm_bufio_shrink_scan() is called. New implementation uses ((LRU size - freed) <= retain_target) condition for scan termination. LRU size can be safely determined inside __scan() because this function is called after dm_bufio_lock(). 2. do_shrink_slab() uses value returned by dm_bufio_shrink_count() to determine number of freeable objects in the slab. However dm_bufio always retains retain_target buffers in its LRU and will terminate a scan when this mark is reached. Therefore returning the entire LRU size from dm_bufio_shrink_count() is misleading because that does not represent the number of freeable objects that slab will reclaim during a scan. Returning (LRU size - retain_target) better represents the number of freeable objects in the slab. This way do_shrink_slab() returns 0 when (LRU size < retain_target) and vmscan will not try to scan this shrinker avoiding scans that will not reclaim any memory. Test: tested using Android device running <AOSP>/system/extras/alloc-stress that generates memory pressure and causes intensive shrinker scans Signed-off-by: Suren Baghdasaryan <surenb@google.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.