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List:       linux-newbie
Subject:    Query - Related to kernel changes for GFP_NOIO flag
From:       Suraj Choudhari <surajschoudhari () gmail ! com>
Date:       2017-03-05 9:19:17
Message-ID: CAGvB2rP=C6AdDw=43pRhS85wbrxcpXE6cypXc=hSdQfJx=q-Fg () mail ! gmail ! com
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Hi,

The GFP_NOIO flag used for kmalloc is currently defined to be '__GFP_RECLAIM'

#define GFP_NOIO        (__GFP_RECLAIM)

#define __GFP_RECLAIM ((__force
gfp_t)(___GFP_DIRECT_RECLAIM|___GFP_KSWAPD_RECLAIM


However, prior to 4.4 kernel, this flag was defined as __GFP_WAIT.
#define GFP_NOIO        (__GFP_WAIT)


Queries -
======
1]  Can any physical disk IO happen due to memory allocation done
using 'GFP_NOIO' ?

      The reason to ask is since the GFP_NOIO is now modified to use
__GFP_KSWAPD_RECLAIM,  so I am thinking that physical disk IO may
probably happen whiel swapping the stale pages to disk in kswapd.

2] If kernel code wants to use kmalloc() and wants to ensure there is
NO any physical/disk IO due to kmalloc,  shall the kernel code use
__GFP_DIRECT_RECLAIM instead of GFP_NOIO ?


Thanks,
Suraj
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