Messages in this thread | | | Date | Tue, 05 Jan 2021 06:57:26 -0800 | From | Chris Goldsworthy <> | Subject | Re: [PATCH] fs/buffer.c: Revoke LRU when trying to drop buffers |
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On 2020-11-24 07:39, Matthew Wilcox wrote: > On Mon, Nov 23, 2020 at 10:49:38PM -0800, Chris Goldsworthy wrote: >> +static void __evict_bh_lru(void *arg) >> +{ >> + struct bh_lru *b = &get_cpu_var(bh_lrus); >> + struct buffer_head *bh = arg; >> + int i; >> + >> + for (i = 0; i < BH_LRU_SIZE; i++) { >> + if (b->bhs[i] == bh) { >> + brelse(b->bhs[i]); >> + b->bhs[i] = NULL; >> + goto out; > > That's an odd way to spell 'break' ... > >> + } >> + } >> +out: >> + put_cpu_var(bh_lrus); >> +} > > ... > >> @@ -3245,8 +3281,15 @@ drop_buffers(struct page *page, struct >> buffer_head **buffers_to_free) >> >> bh = head; >> do { >> - if (buffer_busy(bh)) >> - goto failed; >> + if (buffer_busy(bh)) { >> + /* >> + * Check if the busy failure was due to an >> + * outstanding LRU reference >> + */ >> + evict_bh_lrus(bh); >> + if (buffer_busy(bh)) >> + goto failed;
Hi Matthew,
Apologies for the delayed response.
> We might be better off just calling invalidate_bh_lrus() -- we'd flush > the entire LRU, but we'd only need to do it once, not once per buffer > head.
I'm concerned about emptying the cache, such that those who might benefit from it would be left affected.
> We could have a more complex 'evict' that iterates each busy buffer on > a > page so transforming: > > for_each_buffer > for_each_cpu > for_each_lru_entry > > to: > > for_each_cpu > for_each_buffer > for_each_lru_entry > > (and i suggest that way because it's more expensive to iterate the > buffers > than it is to iterate the lru entries)
I've gone ahead and done this in a follow-up patch:
https://lore.kernel.org/lkml/cover.1609829465.git.cgoldswo@codeaurora.org/
There might be room for improvement in the data structure being used to track the used entries - using an xarray gives the cleanest code, but pre-allocating an array to hold up to page_size(page) / bh->b_size entres might be faster, although it would be a bit uglier to do in a way that doesn't reduce the performance of the case when evict_bh_lru() doesn't need to be called.
Regards,
Chris.
-- The Qualcomm Innovation Center, Inc. The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project
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