Fpage mapping in L4 Pistachio

For inter-process data copying,
I am using Flex page mapping conveniently.

However, I noticed copy failure when the Fpage is large.

My program outline is as follows:
(Code is only explanatory, not real one)

Let me call the mapping side process as Mapper,
and mapped side process as Mappee.

==== (1) Mappee ==================

(1-1) Receive window
Fpage:
mappeefpage: L4_Fpage(0x40000000, 0x100000)
i.e. 1 mega bytes recv window from 1 Giga

==== (2) Mapper ===================

(2-1) Buffer to send/receive data
char xbuf[XSIZE];
// This buffer happened being allocated at 0xA26820.

The Flex page which cover this buffer is used for mapping.
strncpy() is used for string copying.


====== (3) Situation ========

(3-1) WHEN XSIZE <= 6112: No problem

Fpage of Mapper:
mapperfpage: L4_Fpage(0xA26000, 0x2000) with R/W permissions

MapItem:
mapitem: L4_MapItem(mapperfpage, 0)
i.e. send-base is 0.

Fpage is mapped, and data is rightly copyed from Mappee to Mapper.


(3-2) WHEN XSIZE > 6112: COPY FAILED

Fpage of Mapper:
mapperfpage: L4_Fpage(0xA20000, 0x10000) with R/W permissions
// To cover the buffer, Fpage becomes large !
// as the buffer happened to be at 0xA26820 !
MapItem:
mapitem: L4_MapItem(mapperfpage, 0)
i.e. send-base is 0.

==> NO DATA COPIED.

( Cf. 0xA26820 + 6112 == 0xA28000.
&xbuf[6112] is 0xA28000)


===== (4) Question ======

Am I misunderstanding the fpage mapping usage, or
are there some restriction or limitation in L4 flex page mapping ?

Thank in advance.
---
K. Maruyama
National Institute of Informatics, Japan