On Tue, 9 Feb 2010 16:59:26 +0530
Nikanth Karthikesan <knikanth@xxxxxxx> wrote:
> Make vm_max_readahead configurable at run-time. Expose a sysctl knob
> in procfs to change it. This would ensure that new disks added would
> use this value as their default read_ahead_kb.
>
hm, I guess that's useful.
> +int sysctl_vm_max_readahead_handler(struct ctl_table *, int,
> + void __user *, size_t *, loff_t *);
I don't particuarly like the practice of leaving out the identifiers.
They're useful for documentation purposes and it's irritating when you
look at a declaration for some real reason, only to find that the identifiers
were left out.
> int force_page_cache_readahead(struct address_space *mapping, struct file *filp,
> pgoff_t offset, unsigned long nr_to_read);
> Index: linux-2.6/mm/backing-dev.c
> ===================================================================
> --- linux-2.6.orig/mm/backing-dev.c
> +++ linux-2.6/mm/backing-dev.c
> @@ -18,7 +18,7 @@ EXPORT_SYMBOL(default_unplug_io_fn);
>
> struct backing_dev_info default_backing_dev_info = {
> .name = "default",
> - .ra_pages = VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE,
> + .ra_pages = INITIAL_VM_MAX_READAHEAD * 1024 / PAGE_CACHE_SIZE,
> .state = 0,
> .capabilities = BDI_CAP_MAP_COPY,
> .unplug_io_fn = default_unplug_io_fn,
> Index: linux-2.6/mm/readahead.c
> ===================================================================
> --- linux-2.6.orig/mm/readahead.c
> +++ linux-2.6/mm/readahead.c
> @@ -17,6 +17,19 @@
> #include <linux/pagevec.h>
> #include <linux/pagemap.h>
>
> +unsigned long vm_max_readahead = INITIAL_VM_MAX_READAHEAD;
> +
> +int sysctl_vm_max_readahead_handler(struct ctl_table *table, int write,
> + void __user *buffer, size_t *length, loff_t *ppos)
> +{
> + proc_doulongvec_minmax(table, write, buffer, length, ppos);
> +
> + default_backing_dev_info.ra_pages =
> + vm_max_readahead >> (PAGE_CACHE_SHIFT - 10);
> +
> + return 0;
> +}
Hang on. This doesn't only affect newly-added disks. It also affects
presently-mounted filesystems which are using default_backing_dev_info.
xfs, btrfs, blockdevs, nilfs, raw, mtd.
What's the effect of this change? (It should be in the changelog)
> #endif
> -
> + {
> + .procname = "max_readahead_kb",
> + .data = &vm_max_readahead,
> + .maxlen = sizeof(vm_max_readahead),
> + .mode = 0644,
> + .proc_handler = sysctl_vm_max_readahead_handler,
> + },
It'd be nice if the in-kernel and /proc/identifiers were more similar.
That would require that vm_max_readahead be renamed to
vm_max_readahead_kb. We could not bother, I guess. But
vm_max_readahead_kb is a better identifier.
> { }
> };
>
> Index: linux-2.6/Documentation/sysctl/vm.txt
> ===================================================================
> --- linux-2.6.orig/Documentation/sysctl/vm.txt
> +++ linux-2.6/Documentation/sysctl/vm.txt
> @@ -31,6 +31,7 @@ Currently, these files are in /proc/sys/
> - laptop_mode
> - legacy_va_layout
> - lowmem_reserve_ratio
> +- max_readahead_kb
> - max_map_count
> - memory_failure_early_kill
> - memory_failure_recovery
> @@ -263,6 +264,12 @@ The minimum value is 1 (1/1 -> 100%).
>
> ==============================================================
>
> +max_readahead_kb:
> +
> +This file contains the default maximum readahead that would be used.
> +
I think we could provide a more detailed description than this, please.
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