mm: Reduce direct reclaim stalls with RAM-backed swap#610
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The current should_reclaim_retry() code does not account for the fact the number of logical swap pages available for RAM-backed swap (zram, brd) is dependent on having enough free physical pages, and simply always assumes that enough pages are reclaimable to satisfy the allocation. For instance, given a system with a 200GiB zram device (10% used) and 100MB of free physical pages, should_reclaim_retry() incorrectly concludes that it can swap 180GiB worth of anon pages to swap. Because it appears to be always possible to write to swap, the OOM killer is delayed and the system retries in direct reclaim for prolonged periods (20-30 minutes observed in production). Fix this by excluding anon pages from the reclaimable estimate when all active swap devices are RAM-backed. Once file-backed pages are exhausted the watermark check fails and the kernel falls through to OOM as expected. To identify RAM-backed swap devices at swapon time, introduce BLK_FEAT_RAM_BACKED (set by zram and brd) and SWP_RAM_BACKED (swapfile.c). A cached bool swap_all_ram_backed is maintained under swap_lock by swap_update_all_ram_backed() during swapon/swapoff, which is locklessly accessed in should_reclaim_retry(). Signed-off-by: Matt Fleming <mfleming@cloudflare.com>
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Pull request for series with
subject: mm: Reduce direct reclaim stalls with RAM-backed swap
version: 1
url: https://patchwork.kernel.org/project/linux-block/list/?series=1060584