_ore_get_io_state is supposed to allocate a struct ore_io_state, which is
variable length. Setting aside the uglification argument which gave birth
to the patch "pnfs-obj: Uglify objio_segment allocation for the sake of
the principle :-(", the above function checks if the memory it needs to
allocate is bigger than a page, and if so, separates it into two
allocations. why is this done?
from include/linux/slab.h:
>/*
> * The largest kmalloc size supported by the slab allocators is
> * 32 megabyte (2^25) or the maximum allocatable page order if that is
> * less than 32 MB.
> *
> * WARNING: Its not easy to increase this value since the allocators have
> * to do various tricks to work around compiler limitations in order to
> * ensure proper constant folding.
> */
>#define KMALLOC_SHIFT_HIGH ((MAX_ORDER + PAGE_SHIFT - 1) <= 25 ? \
> (MAX_ORDER + PAGE_SHIFT - 1) : 25)
>
>#define KMALLOC_MAX_SIZE (1UL << KMALLOC_SHIFT_HIGH)
>#define KMALLOC_MAX_ORDER (KMALLOC_SHIFT_HIGH - PAGE_SHIFT)
If kmalloc can allocate 32MB, why check one page, like so?
from fs/exofs/ore.c:
>struct __alloc_all_io_state {
> struct ore_io_state ios;
> struct ore_per_dev_state per_dev[numdevs];
> union {
> struct osd_sg_entry sglist[sgs_per_dev * numdevs];
> struct page *pages[num_par_pages];
> };
>} *_aios;
>
>if (likely(sizeof(*_aios) <= PAGE_SIZE)) {
> _aios = kzalloc(sizeof(*_aios), GFP_KERNEL);
> if (unlikely(!_aios)) {
> EXOFS_DBGMSG("Failed kzalloc bytes=%zd\n",
> sizeof(*_aios));
> *pios = NULL;
> return -ENOMEM;
> }
> pages = num_par_pages ? _aios->pages : NULL;
> sgilist = sgs_per_dev ? _aios->sglist : NULL;
> ios = &_aios->ios;
>} else {
> struct __alloc_small_io_state {
> struct ore_io_state ios;
> struct ore_per_dev_state per_dev[numdevs];
> } *_aio_small;
> union __extra_part {
> struct osd_sg_entry sglist[sgs_per_dev * numdevs];
> struct page *pages[num_par_pages];
> } *extra_part;
> _aio_small = kzalloc(sizeof(*_aio_small), GFP_KERNEL);
> if (unlikely(!_aio_small)) {
> EXOFS_DBGMSG("Failed alloc first part bytes=%zd\n",
> sizeof(*_aio_small));
> *pios = NULL;
> return -ENOMEM;
> }
> extra_part = kzalloc(sizeof(*extra_part), GFP_KERNEL);
> if (unlikely(!extra_part)) {
> EXOFS_DBGMSG("Failed alloc second part bytes=%zd\n",
> sizeof(*extra_part));
> kfree(_aio_small);
> *pios = NULL;
> return -ENOMEM;
> }
> pages = num_par_pages ? extra_part->pages : NULL;
> sgilist = sgs_per_dev ? extra_part->sglist : NULL;
> /* In this case the per_dev[0].sgilist holds the pointer to
> * be freed
> */
> ios = &_aio_small->ios;
> ios->extra_part_alloc = true;
>}
Is there any point to check if the memory is greater than 32MB?
--
Idan Kedar
Tonian
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