A search for PageWriteback in 4.9 results in: "undefined identifier", yet a search for PageWriteback in 2.6.25 shows that it is defined there.

Why is PageWriteback in use in 4.9 but I can't find a definition for it?

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    Your title has a question, but the body seems to be two statements. What are you asking? – Jeff Schaller Jul 26 '17 at 18:32
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    @JeffSchaller Why PageWriteback use in 4.9 but can't found define of it? – illiterate Jul 26 '17 at 18:34
  • So you're saying that a particular identifier is in use, but you can't find where it's being defined? – Jeff Schaller Jul 26 '17 at 18:43
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    See if my edit captured your intent, please. – Jeff Schaller Jul 26 '17 at 18:53
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    I think trying to ask for any identifier would be too broad or unclear -- focusing this question on one identifier gives it a chance of an answer (as we've already seen). – Jeff Schaller Jul 26 '17 at 19:01

It is (along with many other PageXXX things) defined in include/linux/page-flags.h, but the definition is obscured by the use of macros. See the macro TESTPAGEFLAG in the above file.

In particular, this definition of the TESTPAGEFLAG macro:

#define TESTPAGEFLAG(uname, lname, policy)              \
static __always_inline int Page##uname(struct page *page)       \
    { return test_bit(PG_##lname, &policy(page, 0)->flags); }

combined with this call to TESTPAGEFLAG with the Writeback parameters:

TESTPAGEFLAG(Writeback, writeback, PF_NO_COMPOUND)

The general question stuck with me, so I thought I'd add an answer as to how I might go about finding an arbitrary identifier in the Linux kernel.

Download a version of the kernel source (or install your distribution's linux-kernel source package), and extract it:

wget https://cdn.kernel.org/pub/linux/kernel/v4.x/linux-4.9.39.tar.xz
tar xf linux-4.9.39.tar.xz
cd linux-4.9.39

Create a minimal .config, just to make a compilable kernel:

make menuconfig # just exit and save

Key step: compile the kernel with the -save-temps gcc flag so that it saves the intermediate pre-processor files:

make KCFLAGS=-save-temps

The compilation process will drop *.i files in the current directory. Find any file where the identifier is defined; I picked the same identifier here, for ease of comparison, but the process would work for any other identifier; I picked zbud.i only because I liked the filename:

grep PageWriteback *.i
zbud.i:static inline __attribute__((no_instrument_function)) __attribute__((always_inline)) int PageWriteback(struct page *page) { return (__builtin_constant_p((PG_writeback)) ? constant_test_bit((PG_writeback), (&({ ((void)(sizeof(( long)(0 && PageCompound(page))))); page;})->flags)) : variable_test_bit((PG_writeback), (&({ ((void)(sizeof(( long)(0 && PageCompound(page))))); page;})->flags))); }

Open the file for viewing, scroll to the identifier in question, then search up/backwards in the file for a line that starts with # to see where the definition came from:

# 255 "./include/linux/page-flags.h"

which points us to the same source file that Nick found before.

Alternatively, you could search for the include lines and/or the identifier, then delete all the trailing lines; the last line of output will be the file where the identifier was defined:

grep -E '^# |PageWriteback' zbud.i | sed '/PageWriteback/,$d' | tail -n 1
# 74 "./include/linux/page-flags.h"
# 108 "./include/linux/page-flags.h"
# 255 "./include/linux/page-flags.h"
  • # 255 "./include/linux/page-flags.h" seem isn't definition of the TESTPAGEFLAG macro.I can't understand you how to found define by use your result. – illiterate Jul 28 '17 at 11:12
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    Nick did an awesome job finding that particular definition; all I did was extend their idea to ("expanding" / combining) the pre-processor tokens to the rest of the kernel so that it'd be easier to find the next one you were curious about. – Jeff Schaller Jul 28 '17 at 15:36

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